All in One SEO Pack – One of the most popular plugins ever for WordPress, this plugin does a bit of everything for you from helping choose the best post title and keywords, to helping you avoid duplicate content and more.

Automatic SEO Links – Automatic SEO Links allows you to choose a word or phrase for automatic linking, both internal and external, set anchor text, choose if it should be “nofollow” or not, and more.  One of the best features of this plugin is that it will only do this for the first occurrence of a word in a post so you don’t have to worry about spamming your post with numerous links to the same thing.

Google XML Sitemaps – An essential tool in any blogger’s armory of SEO tools.  While the name only mentions “Google,” this plugin creates an XML-sitemap that can be read by Ask, MSN and Yahoo also.

HeadSpace2 – This plugin allows you to install all sorts of meta-data, add specific JavaScript and CSS to pages, suggests tags for your posts and a whole lot more.

Nofollow Case by Case – This plugin allows you to strip the “nofollow” command from your comments, and then you can apply it to only the comments you don’t wish to support.

Platinum SEO Plugin – The Platinum SEO Plugin offers you such features as automatic 301 redirects for permalink changes, auto-generation of META tags, post slug optimization, help in avoiding duplicate content and a host of other features.

SEO Title Tag – Your tags are an important part of your site for making sure that search engines know where to place your posts, and SEO Title Tag focuses exclusively on this.  Unlike some other plugins, and WordPress itself, this extension will allow you to add tags to your pages, your main page and even any URL anywhere on your site.

SEO friendly and HTML valid subheadings – Some themes for WordPress will confuse your sub-header tags based on the page they are to be displayed on, but this plugin will automatically reset them to make them more SEO friendly by moving them down one spot in the hierarchical tree.  In other words, h2 becomes h3, h3 becomes h4 and so on.

SEO Friendly Images – Images can be a great source of traffic as people search for images of various subjects, and this plugin helps you with making sure that you have “alt” and “title” tags on all of your images so that the search engines can properly index them.

Sitemap Generator – This is a more customizable sitemap generator than most with options to support multi-level categories and pages, category/page exclusion, permalink support, choices on what to display, options to show number of comments and more.