The best free keyword suggestion tool, in my opinion, is actually more of a mashup than a tool on its own. I'm pretty sure I've seen SEO Book's Keyword Suggestion Tool before, but for whatever reason, it really only struck me as being really useful tonight. I was looking around for any new keyword suggestion tools, and noticed Aaron's. After I ran a sample search on it, I thought, "Wow, this is pretty good stuff".
Much of its appeal is that it makes it easy to get keyword suggestion information from all the major free suggestion tools out there. The resulting table gives links to Keyword Discovery and Wordtracker's free tools, with the search term already supplied via the url. It also includes links for each phrase to Google Trends, Google Traffic Estimator, Google Suggest, Google Synonyms, Adwords Keyword Tool, and Quintura. In addition, it lists the estimated traffic from Google, Yahoo, and MSN.
Again, I'm not sure why it didn't strike me before as being really useful, but for whatever reason, I didn't really notice until now. Better late than never, I guess. This will be my new free keyword suggestion tool of choice.