I have installed, deactivated and removed hundreds of plugins throughout my blogging career. I wanted to share some REALLY useful insights.
1) Google (XML) Sitemaps Generator for WordPress
This plugin generates a XML-Sitemap compliant sitemap of your WordPress blog. This format is supported by Ask.com, Google, YAHOO and MSN Search.
2) All in One SEO Pack
Optimizes your WordPress blog for Search Engines (Search Engine Optimization).
3) Disqus Commenting System
Disqus Comments is a comment system and moderation tool for your site. This service lets you add next-gen community management and social web integrations to any site on any platform. Hundreds of thousands of sites, from small blogs to large publications, rely on Disqus Comments for their discussion communities.
4) WP Greet Box
This plugin lets you show a different greeting message to your new visitors depending on their referrer url. For example, when a Digg user clicks through from Digg, they will see a message reminding them to digg your post if they like it. Another example, when a visitor clicks through from Twitter, they will see a message suggesting them to twit the post and follow you on Twitter. You can also set a default greeting message for new visitors (not matching any referrer URLs) suggesting them to subscribe to your RSS feed.
5) WP Super Cache
This plugin generates static html files from your dynamic WordPress blog. After a html file is generated your webserver will serve that file instead of processing the comparatively heavier and more expensive WordPress PHP scripts. The static html files will be served to the vast majority of your users, but because a user’s details are displayed in the comment form after they leave a comment those requests are handled by PHP.
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.
The WordPress e-Commerce plugin is a state-of-the-art e-Commerce platform with a focus on aesthetics, web standards and usability. It is both free and priceless at the same time.
8) WP Security Scan
Scans your WordPress installation for security vulnerabilities and suggests corrective actions, passwords, file permissions, database security, version hiding, WordPress admin, rotection/security, -removes WP Generator META tag from core code.
9) Secure WordPress
Little help to secure your WordPress installation: Remove Error information on login page; adds index.html to plugin directory; removes the wp-version, except in admin area.
Automatically add links to your favorite social bookmarking sites on your posts, pages and in your RSS feed. You can choose from 99 different social bookmarking sites.