here's the list of all the free SEO tools we use in the course of our work. we originally put it up for our own use to find things quickly but its getting quite popular with the public now.
The great free SEO tool that anyone has to tell is "SEO Book". It provides a wide range of smart and free tools ranging from browser plug-ins to keyword analyzers. They offer best kind of SEO tools available all over the net. http://www.seobook.com Most of the people are using their free tools and are proven to be effective.
Somebody may have already posted this but I always use http://blurple.net/scripts/ Also backlinkwatch is awesome for checking out backlinks yahoo site explorer and google webmasters are good as well bing's webmasters tools can be helpful