WordPress is the best blogging software. In addition to having a solid core with many features, it also includes functionality that allows anyone to improve it through the use of add-ons or plugins. From what I've seen there are hundreds of such plugins and possibly even thousands. The following is a list of the ones that I find the most useful. Do you have a favorite?

404 Notifier
Although you can easily check your web analytics software for web page requests that result in an error, it's easier to just use this plugin to send you a notification via RSS or an e-mail when it happens. This is particularly useful if you've recently changed your URL structure and you want to confirm that your 301 redirect rules are set up correctly. One caveat: It doesn't work unless you're using "friendly" permalinks.

Akismet
By far the most popular spam plug-in for WordPress blogs. The Akismet system uses a central repository that many, many blogs contribute to so that it can identify what is and isn't spam. As a user, you benefit from this analysis when a matching spam comment is submitted to your blog. Flagged items are kept in a database for 15 days in case you want to review them for false-positives. Note that you will need a WordPress API key which you can get for free at WordPress.com.

Bunny's Technorati Tags
Technorati popularized the idea of user-generated tags to classify web content. And while some people may not like the Technorati service, it still is worth the effort of dropping a few tags in to each blog posting because you simply don't know where traffic will come from sometimes. This plugin makes it easy to specify a few tags and can even be used to tag partial RSS feeds.

Contact Form II
This plugin makes it easy to create and configure a contact form. I happen to already have a custom contact form for this site, but I use this plugin on another blog. It takes about a minute to set up.

Custom Posts Per Page
If you want to control how many posts appear on your home page, archive, or category page, this is the plugin for the job. You simply put in the counts for each page type and away you go. It can also be used to simulate a static page on your home page by indicating the home page should have just 1 post.

Dagon Design Sitemap Generator
Sitemaps are important for users and search engines as they offer a single place from which all of your blog's content is accessible. Maintaining a sitemap by hand is tedious and very unnecessary since this plugin will do all the work for you.

Math Comment Spam Protection
This is a simple alternative to the traditional captcha technique of preventing spam. Instead of an image, this plugin displays a simple math question that must be answered before a comment will be accepted.

I used to use the Did You Pass Math? plugin. The end result is the same for each plugin, but the implementation of Did You Pass Math required JavaScript. In addition, if a user needs to hit the back button to correct their answer, the Did You Pass Math plugin would "forget" the entire comment forcing the user to retype it.

Fix Database
A quick and easy way to have old, unused references cleared out of your MySQL database. It will also identify problems with tables although I'm not sure how that part works as my tables have never had a problem.

Google Sitemaps
Similar to the Dagon Design Sitemap Generator, this plugin will generate an sitemap. Two differences with it though are 1) the sitemap must be generated manually by clicking a button and 2) the sitemap is in XML format and is intended for use by search engines (you have to submit it) such as Google, Yahoo, and soon MSN. New sites may get an extra benefit from such a sitemap as search engines may crawl a site sooner than they would have otherwise.

PageDate 2
For some reason, pages (not posts) in WordPress do not have their update modified when changes are made. This plugin fixes that oversight and it does it automatically.

Remove Index.php
If you're running WordPress on Microsoft's Internet Information Server (IIS) and want to use friendly URLs, this is a good solution. It's not perfect, but unless you have access to your web server to install an extension such as IIS Rewrite or ISAPI Rewrite, this plugin is your best bet.

Similar Posts
It is typical to indicate the category that a post belongs to so that users have a way to explore related content on your site. An additional technique involves listing specific posts that are related to the current one. This plugin does just that. For an example of what it looks like, check out the links at the lower right of this page.

Social Bookmark Creator
The value of including social bookmark links at the bottom of posts is questionable. Some argue that including the links acts as a prompt for users to bookmark the page. Others argue that anyone that regularly uses a bookmarking service already a browser extension installed making the web page links redundant. I figure it can't hurt to have them. Check them out at the bottom of this page.

Subscribe To Comments
I don't know about you, but I often comment on blogs and then forget to re-visit them to see if there's been a follow-up. With this plugin, users can check a box when they're making a comment and then rest assured that they will be informed of updates to the thread.

WordPress Database Backup
Like insurance, a backup can minimize the impact of a disaster. Don't lose your blog content just because you were too lazy to click the backup button that this plugin provides!

Word Statistics
If you're interested in readability statistics, you'll like this plugin. I don't care about such information, but I do like knowing how many words a post has and this plugin gives that to me.