HitTail - This tool reveals in real time the most promising yet least utilized keywords that drive traffic to your site organically.
Link Hub Finder Tool - This tool will help you find the hub sites for any niche/category. BTW, you need a Google API Key to use this tool.
Quintura - Use this tool to figure out where you live in your "keyword neighborhood". Quintara is a search engine that displays your results both visually and as a standard list.
Buzz- A keyword research tool that will tell you who's talking about a certain keyword.
SummitTOOLS Spider Simulator - This tool tells you how a search engine spider reacts to your pages and what can be done to boost your visibility and subsequent search engine ranking.
SEOBook Keyword Suggestion Tool - This tool combines the results from or links to all the useful SEO keyword research resources out there into one single interface.
Virante Keyword Theme & Link Relationship Tools - Submit a single keyword and it will serve up a list the keywords most often found in conjunction with your keyword in the content of all the sites on the World Wide Web.
Microsoft AdCenter Labs Search Funnel - The Search Funnel tool is for those of you who are interested in visualizing how your average Joe uses a search engine.
Google Webmaster Central - One-stop shop for from-the-source information about how Google crawls and indexes websites.
Stephan Spencer's Blog and RSS Feed Search SEO - Tips and tactics for optimizing your blogs and RSS feeds for SEO purposes. (You can also search Google for "blog optimization" and click on the number one listing, that's TopRank!)
Linkable Tool - This tool lists all the factors that go into Phillip Lessen's good-or-not-good filter producing a score and an explanation as to why or why not your blog posts are linkable.
Mobile Search Marketing - Repository of tips and tricks on the nascent topic of mobile SEO.