Here is a list of a few good WordPress plugins.
UPDATE: This post is really dated now and while I might still recommend some on this list, I’ll be updating this post soon with all new recommendations.
Some WordPress plugins duplicate each other so be selective, understand what the plugin does BEFORE activating and only activate those you really use. Backup your database before activating.
Out-of-the-box SEO for your WordPress blog.
Checks your blog for broken links and missing images and notifies you on the dashboard if any are found.
Saves your wordpress blog from getting tagged as ping spammer. (Note: this plugin is a fork of the MaxBlogPress Ping Optimizer plugin, with registration/activiation functionality removed.)
A tag cloud plugin for WordPress to give you more flexibility with the styling of your tag cloud.
One of the nice things about contact form 7 is that it integrates with Akismet. This integrations will significantly reduce the amount of contact form spam by checking the author name and email against the Akismet database.
Replaces the default WordPress editor with CKeditor
With this plugin, you can safely change your permalink structure without breaking the old links to your website,and even doesn’t hurt your google pagerank.
Each time you create a new post it shows a list of possible pingback enabled blogs with relevant contents to your blog entry. You can then choose ones you will ping back and it will do this automatically. It also collects statistics about pinged blogs and relevant information so future blog posts will show even more relevant blogs to ping back to.
Easy Contact is a simple, semantic contact form that utilizes the Sandbox design patterns. Insert using [easy-contact]. A plaintxt.org experiment for WordPress.
Standards compliant FLV embedding in your blog posts using SWFObject by Geoff and FLV Player by Jeroen. Supports Video Sitemap generation.
Free Traffic Getting SEO PluginTM 2.0 finds the best SEO plugins available for wordpress and then optimizes their settings so you don’t have to. Saves you time and helps get you more traffic.
GD Press Tools is a collection of various administration, seo, maintenance and security related tools that can help with everyday blog tasks and blog optimizations.
This plugin will generate a special XML sitemap which will help search engines like Google, Yahoo, Bing and Ask.com to better index your blog.
Meta-data manager on steroids, allowing complete control over all SEO needs such as keywords/tags, titles, description, stylesheets, and many many other goodies.
Allows you to point WordPress pages or posts to a URL of your choosing. Good for setting up navigational links to non-WP sections of your site or to off-site resources.
Automatically creates a virtual robots.txt file for your blog.
Complete SEO solution for your WordPress blog.
Automatically installs WordPress plugins by name and URL. Provides automatic update of multiple plugins.
Does nothing by itself but supplies common code for the Similar Posts, Recent Posts, Random Posts, and Recent Comments plugins. Make sure you have the latest version of this plugin.
Really Simple CAPTCHA is a CAPTCHA module intended to be called from other plugins. It is originally created for my Contact Form 7 plugin.
Manage all your 301 redirects and monitor 404 errors
Automatically adds alt and title attributes to all your images. Improves traffic from search results and makes them W3C/xHTML valid as well.
SEO Smart Links provides automatic SEO benefits for your site in addition to custom keyword lists, nofollow and much more. (Brian Edition)
Use your blog comments to get more traffic.
Search engine optimize your blog’s title tags. Create a customized title tag for any post, static page, category page, UltimateTagWarrior tag page – indeed, any URL! And will auto-insert the tag name into the title on tag pages generated by UltimateTagWarrior plugin. Admin allows for mass editing of title tags.
Extended Tagging for WordPress 2.8 and 2.9 ! Suggested Tags, Mass edit tags, Autocompletion, Tag Cloud Widgets, Related Posts, Related Tags, etc!
Rescue your viewers from site errors! When content cannot be found, Smart 404 will use the current URL to attempt to find matching content, and redirect to it automatically. Smart 404 also supplies template tags which provide a list of suggestions, for use on a 404.php template page if matching content can’t be immediately discovered.
Improved real time stats for your blog
Find and suggest tags and photos (from Flickr) for your content. Integrates with the Calais service.
Retweet Button Adds a button which easily lets you retweet your blog posts.
A complete integration between your WordPress blog and Twitter. Bring your tweets into your blog and pass your blog posts to Twitter. Show your tweets in your sidebar, and post tweets from your WordPress admin.
Bit.ly URLs Use Bit.ly for URL shortening with Twitter Tools. This plugin relies on Twitter Tools, configure it on the Twitter Tools settings page.
Exclude posts in certain categories from being tweeted by Twitter Tools. This plugin relies on Twitter Tools, configure it on the Twitter Tools settings page.
Set #hashtags for blog post tweets sent by Twitter Tools. This plugin relies on Twitter Tools, configure it on the Twitter Tools settings page.
WordPress plugin to analyze your visitors traffic with real time stats, chart and a lot of chronological informations. It has sidebar Widget support to show current online visitors and other statistics. For WordPress 2.2 or higher.
On-demand backup of your WordPress database. Navigate to Tools ? Backup to get started.
Blocks suspicious-looking requests to WordPress.
Show your mobile visitors a site presentation designed just for them. Rich experience for iPhone, Android, etc. and clean simple formatting for less capable mobile browsers. Cache-friendly with a Carrington-based theme, and progressive enhancement for advanced mobile browsers.
Renames WordPress tables to mitigate some SQL injection attacks.
W3 Total Cache improves the user experience of your blog by improving your server performance
AdServer for WP
Manages your WordPress database. Allows you to optimize database, repair database, backup database, restore database, delete backup database , drop/empty tables and run selected queries. Supports automatic scheduling of backing up and optimizing of database.
An extremely powerful anti-spam plugin that virtually eliminates comment spam. Finally, you can enjoy a spam-free WordPress blog! Includes spam-free contact form feature as well.
Automatically finds tags based on your post content.
Perform security scan of WordPress installation.
Very fast caching plugin for WordPress.
A plugin which formats your site with a mobile theme for the Apple iPhone / iPod touch, Google Android, Palm Pre and other touch-based smartphones.