BizGrowth has put together a control panel of SEO tools. The site lists and categorizes over 150 free online SEO related tools, ranging from keyword analyzers and meta tag generators to page rankers and site analyzers, with some miscellaneous tools thrown in as well.
I've used most of these tools before, but there are some I've never encountered, so it's always fun to try new SEO tools. I like that each link opens the tool in a new window (or tab), so I can easily go back to the control panel to try another.
Collections of related things, whether those things are tools or something else, is always nice (and link-worthy). Plus, it makes it easy to go back and find a tool you've used before, but forgot to bookmark. Nice...