IMHO, All in one SEO is by far the best Search engine optimization plugin available for Wordpress.

The most important factors for a good search engine indexing :

• Titles
  The text that you see in your browser’s window bar is the most important thing *on* your page. Make sure your titles are getting rewritten and the important stuff (your post titles) always come before anything else. You can leave a few words from your blog title to do some branding but don’t overdo. If your post titles don’t contain your most precious phrases you want to rank for then tweak them. Leave the post titles as it is but change the meta title.
• Descriptions
  Once your pages rank in search engines your description (aside of course from your title) makes a potential visitor want to click through, or not. It’s mta descriptions, believed dead by many people, that decide over your site’s fate. If you don’t want to invest the time and write a description for every post at least let them be auto-generated. This vastly better than the usual “Share This … Posted on … under …” description that is often used. If a post has an excerpt it is used, but not if you specify an explicit description.
• Keywords
  All in One SEO Pack can generate them from your categories (this was the old way of tagging your posts in wordpress) or from Ultimate Tag Warrior or, starting with Wordpress 2.3, the built-in tagging system. Using your categories is optional, the rest is auto-detected and used if there.

Another feature I like about All in One SEO Pack is that it gives you the option to make a META-TITLE tag, which can be the single most important factor when a search engine indexes your page.

Download it now !

Source : Uberdose 2.0

PS : theblogjoint.com has a compilation of a list of good SEO plugins for Wordpress. Do check that out too.