Here is a list of the most important on-page ranking factors and how they are addressed by this All in One SEO plugin. You might want to visit Options - All in One SEO to tweak the default options.

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.

Duplicate Content

You can have “noindex” and the like generated for archive-, category- and tag pages if you seem to have trouble with duplicate content.

Pricing: Free
Requirements: -
Source: http://wp.uberdose.com/2007/03/24/all-in-one-seo-pack/