User Experience
Extended Live Archive - implements a dynamic, AJAXified way of digging into the archives of a blog.
Feedburner Feed Replacement - This popular plugin was recently adopted by Feedburner and renamed “FeedSmith†. This plugin forwards all of your feed traffic to Feedburner. FeedSmith often causes an elusive “bump in subscribers†when you first activate it because it will detect all ways to access your feed. Readers coming from http://www.yoursite.com/feed/ or http://www.yoursite.com/wp-rss2.php will both be redirected to your FeedBurner feed so that you can accurately track your subscriber base.
Related Posts - Returns a list of the related entries based on active/passive keyword matches. Increase pageviews by providing targeted related content.
Subscribe To Comments - Allows readers to recieve notifications of new comments that are posted to an entry.
Brian’s Threaded Comments - adds a ‘Reply to this comment’ link to every comment. Makes it easy for people to reply to comments and discuss better.
Landing Sites - When visitors is referred to your site from a search engine, it shows them related posts to their search on your blog.
OpenID Comments for Wordpress - allows users to leave comments using their OpenID.
Popularity Contest - keeps a count of your post, category and archive views, comments, trackbacks, etc. and uses them to determine which of your posts are most popular.
Related Posts - This plugin will find other blog posts that are related to the current post based on keyword matching. You can then display the related posts at the bottom of each article. This is an excellent way to keep visitors at your site for longer periods of time. Customize this plugin to display as many or as few related posts as you desire.
WordPress Mobile Edition - shows an interface designed for a mobile device when visitors come to your site on a mobile device.
Social Marketing
Share This - lets your visitors to add your post to various social bookmarking sites, or send a link via e-mail to a friend.
Delicious Cached++ - uses the built-in MagpieRSS engine functions to create a list of the latest bookmarks on a del.icio.us account, from its RSS feed, keeping the feed cached in the database.
WP-Email - Allows people to recommend/send your WordPress blog’s post/page to a friend.
Management
WordPress Database Backup - On-demand backup of your WordPress database. Back up your blog today.
Exec-PHP - allows php tags inside the content or excerpt of your posts and pages to be executed just as in usual PHP files.
Photo Dropper - lets you add free photos (Flickr / Creative Commons) to your blog posts without leaving Wordpress.
Wordpress Reporter - allows you to easily view your Google Analytics and Feedburner reports in your WordPress administration area.
Wordpress Database Backup - Talk about a lifesaver. This plugin comes installed with Wordpress, providing an easy way to backup your Wordpress database. You can download the backup file or have it emailed to the address of your choice.
Advertising
Adsense Deluxe - offers advanced options for managing the automatic insertion of Google AdSense or Yahoo Publisher Network (YPN) ads to your WordPress posts. Easily switch all AdSense ads to a new color scheme across the entire site.
AdSense Manager - is a Wordpress plugin for managing AdSense ads on your blog. It generates code automatically and allows positioning with Widgets.
RSS
KB Advanced RSS Widget - a wordpress widget that gives you complete control over how RSS feeds are parsed for your sidebar.
Feedburner Feedsmith - Forwards all feed traffic to Feedburner while creating a randomized feed for Feedburner to pull from.
Simple Feed Copyright - adds a simple copyright notice to full text wordpress feeds.
RSS Signature - allow the addition of a custom tagline, signature or copyright message to the wordpress generated rss feeds.
Spam
Akismet - Is a spam filter that checks your comments against the Akismet web service to see if they are spam or not, also checks the trackbacks for spam.
Spam Karma 2 (SK2) - is an anti-spam plugin for the WordPress meant to stop all forms of automated Blog spam effortlessly, while remaining as unobtrusive as possible to regular commenters.
Bad Behaviour - This plugin isn’t as well known as the others but it should definetly be on your list. Bad Behaviour denies automated spambots access to your PHP-based Web site. This plugin complements Akismet by preventing spammers from ever delivering their junk in the first place.
- Defensio is a spam filtering web service to block comment spam.
SEO
Google Sitemaps - This generator will create a Google compliant sitemap of your WordPress blog.
Google XML Sitemaps Generator - creates a sitemap of all articles on your WordPress blog. Helps Google and other search engines index every article you write.
Quick META Keywords - will automatically add META Keywords tags to every single post based on categories under which the article is categorized.