WordPress Plugins allow easy modification, customization, and enhancement to a WordPress blog. Instead of changing the core programming of WordPress, you can add functionality with WordPress Plugins. Here is a basic definition:

WordPress Plugin: A WordPress Plugin is a program, or a set of one or more functions, written in the PHP scripting language, that adds a specific set of features or services to the WordPress weblog, which can be seamlessly integrated with the weblog using access points and methods provided by the WordPress Plugin Application Program Interface (API).

Here is a great list of the most useful and most popular WordPress plugins,Enjoy the list.

Google XML Sitemaps

This plugin will create a Google sitemaps compliant XML-Sitemap of your WordPress blog.

All in One SEO Pack

Automatically optimizes your WordPress blog for Search Engines (Search Engine Optimization). This plugin also helps you avoid duplicate content found in blogs. With this pack, you can easily modify and tweak everything.

WordPress Database Backup

WordPress database backup creates backups of your core WordPress tables as well as other tables of your choice in the same database. setups your backups to occur at almost any imaginable frequency.

Contact Form 7

Just another contact form plugin. Simple but flexible.
Contact Form 7 can manage multiple contact forms, plus you can customize the form and the mail contents flexibly with simple markup. The form supports Ajax-powered submitting, CAPTCHA, Akismet spam filtering and so on.

Google Analytics for WordPress

The Google Analytics for WordPress plugin automatically tracks and segments all outbound links from within posts, comment author links, links within comments, blogroll links and downloads. It also allows you to track AdSense clicks, add extra search engines, track image search queries and it will even work together with Urchin.

PXS Mail Form

Creates a mail form with multipart verification, various messages and an auto redirect on successful send. It is much more easy to contact with website author.
The current download is a new version that has some neat new features and better compatibility. It has a CC Sender option and no longer requires a seperate admin options file.

WP e-Commerce

The WP e-Commerce shopping cart plugin for WordPress is an elegant easy to use fully featured shopping cart application suitable for selling your products, services, and or fees online.
WP e-Commerce is a Web 2.0 application designed with usability, aesthetics, and presentation in mind.

ShareThis

The ShareThis WordPress plugin provides a simple way for users to add your post to many social bookmarking sites, or to send a link to your post via email, AIM, Facebook, Twitter and more.

Redirection

Redirection is a WordPress plugin to manage 301 redirections, keep track of 404 errors, and generally tidy up any loose ends your site may have. This is particularly useful if you are migrating pages from an old website, or are changing the directory of your WordPress installation.
bad SEO is caused by broken links and indexing problems. Redirection enables you to prevent Google Bots and Yahoo Spiders locking themselves in a loop. This plugin improves your site indexing and navigation by allowing you to create simple 301 redirects in order to avoid those pesky indexing problems.

Yet Another Related Posts Plugin

Yet Another Related Posts Plugin (YARPP) gives you a list of posts and/or pages related to the current entry, introducing the reader to other relevant content on your site.

Sidebar Widgets

It’s called Sidebar Widgets because it allows you to move things (widgets) around, in and out of your sidebar. A widget is something that you might want on your sidebar, such as a category list or the most recent comments or a link to your admin pages. There is even a widget that lets you type whatever you want in the sidebar, such as some HTML or Javascript code.

Get Recent Comments

This plugin displays excerpts of the most recent comments and/or trackbacks that have been posted to the articles in your blog.

Akismet

Akismet checks your comments against the Akismet web service to see if they look like spam or not and lets you review the spam it catches under your blog’s “Comments” admin screen.

WP lightbox 2

Overlay images on the current page for WordPress. You can turn it on and all images are now working with it.

Ultimate Tag Warrior 3

Ultimate Tag Warrior is a multi-headed hydra of a plugin. It allows you to add tags either through the Write Post page in wordpress in a tag box, on posts using an AJAXy box, and in posts using special syntax from external editors (or internally, if you’d like). From the write post page, you can also get suggestions for tags using the Yahoo! keyword suggestion service.Once you’ve got your tags entered; you enter a realm of interesting ways of doing things with your tags. At the complicated end, AJAXy search is included with the download (it may need some customisation to match your theme). At the simpler end, you can automagically have tags included at the top and bottom of your posts without making changes to themes.

My Page Order

My Page Order allows you to set the order of pages through a drag and drop interface. The default method of setting the order page by page is extremely clumsy, especially with a large number of pages.

Adsense-Deluxe

AdSense-Deluxe is an easy-to-use plugin for WordPress 1.5+ (including WP 2.0) for quickly inserting Google or Yahoo! ads into your blog posts, and managing when and where those ads are displayed. It was developed for my own use after an exhaustive search for a similar tool turned up nothing adequate.

Search Regex Plugin

Search Regex adds a powerful set of search and replace functions to WordPress. These go beyond the standard searching capabilities, and allow you to search and replace almost any data stored on your site. In addition to simple searches you have the full power of PHP’s regular expressions at your disposal.

Tweetmeme

The TweetMeme button plugin allows you to easily integrate the TweetMeme button into your WordPress blog. The plugin will put the button into both your content and your feed.

WP-SpamFree Anti-Spam

An extremely powerful WordPress anti-spam plugin that eliminates blog comment spam, including trackback and pingback spam. Finally, you can enjoy a spam-free WordPress blog!