http://www.mattcutts.com/blog/free-direct-text-links/

http://www.google.com/support/webmasters/bin/answer.py?&answer=93641

http://www.google.com/support/webmasters/bin/answer.py?answer=100044&cbid=-6o2s9sgm6no7&src=cb&lev=answer

http://codex.wordpress.org/Creating_an_Error_404_Page

http://www.codinghorror.com/blog/archives/000819.html

http://teqsnacks.com/2007/08/11/wordpress-turning-404-not-found-random-visitors-to-blog-readers/

http://www.gravytrain.co.uk/blog/2009/07/10/how-to-manage-broken-links-and-404-error-pages/

http://www.spotonseoservices.com/custom-404-error-pages-site-usability/

http://www.goldseo.com/news/customize-word/

Some examples of 404 error pages

http://andrewferguson.net/wordpress-plugins/dunstan-style-error-page/

http://www.blogcatalog.com/search.frame.php?term=error+404+wordpress&id=adac469ca34fa44577c7f945d788a294

http://www.blogcatalog.com/search.frame.php?term=error+404+wordpress&id=c154e5fa70de5b126287f8bd9e2c87af

BROKEN LINK REMOVER AND FIXER plugins

http://www.dailyseoblog.com/2009/06/8-wordpress-plugins-to-make-your-404-error-pages-user-friendly-efficient/

http://techblissonline.com/broken-links-remover-wordpress-plugin/

broken-links-remover.zip

http://blog.semetrical.com/link-building-using-404-error-reports/ makes use of 404 for link building!!

Fixing Permalink Problems
Fixing .htaccess Generation Issues
If your installation of WordPress does not generate a .htaccess file or if it does not write the new rules onto your existing .htaccess file then there are a couple reasons that could be causing this. Work step by step and continue to the next step only if the previous step does not work.

Change File Permissions: You must chmod the .htaccess file to 666 to edit it with the WordPress template editor, but this is not recommended, since if you do that, any user of your blog, who can edit templates will be able to edit it. You can change the permissions to 660 to make it server-writable, which again will have the same limitation.
Server Blockage: Your host might have blocked the SERVER_SOFTWARE variable and this will cause WordPress’ .htaccess generation to fail. If you are sure that your server is running Apache, you can force WordPress to believe that your server is running Apache by changing your wp-includes/vars.php file. Follow the steps below to implement these changes.
Open the wp-includes/vars.php file using the built in file editor in your WordPress Admin panel.

To navigate to this panel, login to WordPress, click on “Manage”, then on “Files”, scroll to the bottom and type in wp-includes/vars.php into the text box under the “Other Files” title.
Look for
$is_apache = strstr($_SERVER['SERVER_SOFTWARE'], ‘Apache’) ? 1 : 0;
and replace it with
// $is_apache = strstr($_SERVER['SERVER_SOFTWARE'], ‘Apache’) ? 1 : 0;
Add a new line under
// $is_apache = strstr($_SERVER['SERVER_SOFTWARE'], ‘Apache’) ? 1 : 0;
and type in
$is_apache = 1;
Users of XAMPP (Windows): Some versions of XAMPP do not enable mod_rewrite by default (though it is compiled in Apache). To enable it — and thus enable WordPress to write the .htaccess file needed to create pretty permalinks — you must open apache/conf/httpd.conf and uncomment the line LoadModule rewrite_module modules/mod_rewrite.so (i.e., delete the hash/pound sign at the front of the line).
Permalinks, .htaccess, and MS Frontpage
A note about Microsoft Frontpage: many servers (shared and dedicated) maintained and built by various hosting companies come with mod_frontpage compiled with the apache build, and in many cases with the Frontpage Server Extensions installed, on each virtual server. This is more common than not, many/most binary distributions used in the server build process at most hosting companies these days include both mod_fronpage and the server extensions. Even if you’re not using Frontpage, because of the way that the extensions interact with apache (and the httpd.conf file) you’ll likely get something like a 500 error or blank white page when trying to view your WP install (although the admin panel may operate correctly) simply because extensions/mod_frontpage exist on your server.

Wordpress will operate correctly with the Frontpage Extensions installed, however permalinks will not function at all and ANY change to the permalinks section from the Wordpress admin interface will cause corruption of the Frontpage server extensions due to the addition of the mod_rewrite rules to the .htaccess file. There is however now a fix for this situation.

Quick Fixes, Frontpage or Permalinks
Frontpage Extensions Fix: If you don’t care about permalinks and just want to make the MS Frontpage server extensions work again, simply edit your .htaccess file and remove the WordPress section with the rewrite rules.

To Use Permalinks: If you don’t care about Frontpage(but your hosting company has the extensions installed)

You will need to remove (or have your hosting company do so) the MS Frontpage server extensions, or simply edit the .htaccess file to removed all of the Frontpage Lines, leaving only the WordPress mod_rewrite code.

Using FrontPage AND Permalinks Together
Finally, A solution.

There have been a number of threads on this issue in the support forums, and until now, no solution to the problem.

Normally, on a Unix server with the Microsoft FrontPage Server extensions installed WordPress works just fine and you are able to edit and publish pages (with Microsoft FrontPage) — until — you make a change to the permalinks (for example to the date based kind that I like /2005/04/etc). I often suggest that type of URI to folks asking about permalinks etc, as that is the method recommended by the w3c (see http://www.w3.org/Provider/Style/URI ).

Now, the problem is that FrontPage uses the .htaccess file (which the WordPress mod_rewrite rules must access) for its “publishing” and “web authoring” configuration. As soon as the WordPress mod_rewrite code is added to the file, two things happen — the permalinks don’t work and the Frontpage Server extensions become corrupt.

I have tried countless ways to get around this, including trying to use rewrite rules that “ignore” the %{HTTP_USERAGENT)% used by FrontPage, to using a second AccessFilename .wpaccess to the httpd.conf file, and a host of other things, and nothing worked to allow use of FrontPage and management and use of permalinks in WordPress at the same time.

The solution is actually simple, and I figured it out by accident.

If you are using, or wish to use FrontPage (or if your hosting package is pre-configured that way) along with WordPress, you’ll need to take the following simple steps on your server or have your hosting company do them for you.

Microsoft FrontPage creates the following directory

_vti_bin
Nested within that it creates both
_vti_adm
and
_vti_aut
In addition to in your site (or WordPress) root folder in all of those directories you will find additional .htaccess files.

In all three of these directories AND in the root directory, at the top of ALL of the .htaccess files you simply need to add one line:

Options +FollowSymlinks
There may or may not already be a line in each like

Options None
Edit and save each .htaccess file and you’re done. Now everything works perfectly, including FrontPage, AND the permalinks of your choosing.

Long Permalinks
When using extra long permalinks in email and posting in comments and chats, some long permalinks are “chopped off” or only the first section is actually recognized as a link and the end seen as text. Here is an example.

http://yourdomain.example.com/2005/10/4/article-about-joe-fred-sally-and-bog
Can result in:

http://yourdomain.example.com/2005/10/4/article-about-joe-fred-sally-and-bog
To click on the lower link, the user would get a 404 Page Not Found Error. If you have a tendency to use very long permalink post titles, take these steps to prevent this problem.

1. Check that you are indeed using Permalinks.

2. Edit your .htaccess file and add the following:

RewriteRule ^post/([0-9]+)?/?([0-9]+)?/?$ /index.php?p=$1&page=$2 [QSA]
3. Test it. Find a post’s ID number and type the following (with your information) in your browser and you should be redirected to your post:

http://yourdomain.example.com/post/(the ID #)
It is also worth noting that most email software will not cut off URLs that have been delineated with angle-brackets (), so when pasting URLs into emails, you should write them as so:

Read my blog post at
Additionally, some decent email clients offer a “preformat” option when composing plain-text emails. Using the “preformat” option when pasting links will force the email client not to insert linebreaks inside the links.

Fixing Other Issues
If your .htaccess file is being generated correctly, but Permalinks still do not function, the following might be a problem. If problems persist, post a note in the WordPress Forum’s How To section.

AllowOverride Not Enabled
Your server may not have the AllowOverride directive enabled. If the AllowOverride directive is set to None in your Apache httpd.config file, then .htaccess files are completely ignored. In this case, the server will not even attempt to read .htaccess files in the filesystem. When this directive is set to All, then any directive which has the .htaccess Context is allowed in .htaccess files. Example of enabled AllowOverride directive in httpd.config:

Options FollowSymLinks
AllowOverride All

You may also have to enable the AllowOverride directive in your DocumentRoot:

# … other directives…
AllowOverride All

You may also have to change the AllowOverride settings for the site. This is surely the case when using Mac OS X Server, but might be likewise with other systems. Usually you can find the site configuration files in /etc/httpd/sites/
If you don’t want to set AllowOverride to all (as it is above) then your AllowOverride list must include the FileInfo directive. You must restart your Apache server for any httpd.config file changes to take effect. For more information on which overrides are allowed, read about Apache Core Features.
Paged Navigation Doesn’t Work
Sometimes navigation to second (and subsequent) pages of posts does not work as expected. Your page may generate a link to a page with one of these URIs:
http://www.example.com/page/2/
http://www.example.name/category/categoryname/page/2/
http://www.example/year/month/day/page/2/
http://www.example/year/month/page/2/
The result of clicking one of those links is that the page loads with all the surroundings (header, footer, sidebar), but instead of a page of posts, there is an error message: “Sorry, no posts match that criteria.”
This is due to a glitch in the .htaccess file that WordPress generates. To fix it, delete the contents of your .htaccess file and re-create it.
In the Control Panel, go to Manage > Files (More Info on Editing Files)
Click the link to your .htaccess file to edit its contents
Copy the contents of the file and paste it to a text file in a text editor. This is a precaution in case your .htaccess file has manual entries for redirects, denials or other handy htaccess tricks
Delete all contents from your .htaccess file and click the Update File button.
In the Control Panel, go to Options > Permalinks.
Click the Update Permalink Structure button to freshly generate new rewrite rules for your permalinks.
Test the results using a link that had previously broken.
Add any manual htaccess entries back in your file (Place manual htaccess entries before the # BEGIN WordPress or after # END WordPress lines.)
You may also perform similar steps by deleting the .htaccess files from the server, creating a fresh empty .htaccess file, changing its permissions to 666, and then in Options > Permalinks generate a new set of htaccess rules by clicking the Update Permalinks Structure button.
If that still doesn’t work, take a look at the WordPress support forums, specifically, this support post.
Permalinks to pages don’t work
If you’ve tried to navigate to a newly created Page and encounter an error, you likely need to update your Permalink structure. Remember, each time you add a new static Page to WordPress, new rules must be generated and updated to .htaccess (WordPress 1.X) or to the internal rewrites array (WordPress 2.X).
Permalinks to Ultimate Tag Warrior tag pages don’t work
If you get 404 errors on local tag URLs when using the UltimateTagWarrior plugin on WordPress 2.X, it’s because the internal rewrites generated by WordPress are being overly greedy and getting invoked before UTW’s rewrite rules have a chance. This usually occurs only when using a custom permalink structure (like /%postname%/). To fix it, either switch your Permalink structure to “Date and name based” or hack the UTW plugin to place the UTW rewrites at the top of the internal rewrites array. More Info on this.
Permalinks work but no pages are returned
Some versions of PHP 4.4.x and 5.x have a bug that causes mod_rewrite to fail when used with some versions of Apache 2.x. More details at http://bugs.php.net/bug.php?id=35096 and http://bugs.php.net/bug.php?id=35059.
More Help
If these steps do not work, search for your problem in the Codex, Troubleshooting, or in the Support Forum. As a last resort, file a bug report.

Tips and Tricks
If you want to list your site in Google News, one of the requirements is to have at least three digit identifier at the end of every url.

This is easily accomplished using %postname%-%post_id% structure, which will append post ID to the end of URL so it becomes http://mysite.com/cooking-tips-tricks-344.

Having your posts end in .html
There’s an easy way to having your posts end in a .html extension, using the structure tags above. Following the example used on properly terminating permalinks, you could have a page like http://yoursite.com/2006/01/01/happy-newyear.html with this rule:

/%year%/%monthnum%/%day%/%postname%.html
Note that this does not generate static .html files. It only adds the .html extension, pages are still being dynamically generated. The SEO benefit to this is debatable, but it can be useful should you need to migrate away from WordPress, since the pages can easily be made static and retain their URL structure.

WordPress versions prior to 2.3 lacked canonical URLs, making .html something very beneficial to add (forcing the URL to be canonical). Now it only provides limited, if any SEO benefits (see External Resources for further analysis).

Avoiding interpretation as an archive link
Note that even though one might never make more than one posting a day, and thus wishes to use e.g., %year%%monthnum%%day%, links so generated will however be interpreted as the archive of all posts for that day. One needs at least %year%%monthnum%%day%%hour% to target an individual post.

See Also
Linking Posts Pages and Categories for how to link from one article to another
External Resources
Optimize WordPress Permalinks
Beautify your URLs with Permalinks
How to Configure WordPress to Create Search Engine Friendly URLs for Permalinks
The htaccess Rules for all WordPress Permalinks
URL Rewriting Tutorial — the whats, hows and wherefores.
Boost Search Engine Optimisation (SEO) using Permalinks
Understanding WordPress Permalinks
WordPress Permalink Canonicalization using htaccess
Customize and Optimize Your Dated WordPress Permalinks
Clean Permalinks for IIS using .htaccess
The Easiest Lighttpd Rewrite Rule
URL rewriting with lighttpd
Permalinks with Lighttpd — using mod_magnet; works on 1.4.2+