It used to be that there were just two plugins everyone used for SEO in WordPress: All in One SEO, and the Ultimate Tag Warrior. Times have changed and these two are no longer at the of the heap in my book. I like having as few plugins as possible and so I regularly search for the best solution. Here’s my three favorite SEO plugins for 2010, and these do not overlap in functionality.
 
Greg’s High Performance SEO
Configure over 100 separate on-page SEO characteristics. Load just 600 lines of code per page view. No junk: just high performance SEO at its best.
http://counsellingresource.com/features/2009/07/23/high-performance-seo/
 
Keyword Statistics
The Keyword-Statistics-Plugin checks the content of a post or a page for keyword-density (single words and optionally 2- and 3-word phrases; for each the 1-10 most commonly used can be displayed). It can update its informations automatically while the author is writing his content in a variable interval (every 1-10 seconds) or manually by clicking on a button. The script comes with english and german stopwords, which optionally can be filtered out before calculating the keyword-densities. Moreover the commonest keywords are extracted in a list as a meta-keyword suggestion. Based on this list a description can be created and automatically set.
http://www.keyword-statistics.net/wordpress-plugin.html

Simple Tags
Extended Tagging for WordPress 2.8 and 2.9 ! Suggested Tags, Mass edit tags, Autocompletion, Tag Cloud Widgets, Related Posts, Related Tags, etc! No nonsense and get’s the job done.
http://redmine.beapi.fr/projects/show/simple-tags