Webdev,
For paid ones, I hear good things about wordtracker.com and keyworddiscovery.com
But these are kinda pricy.
For free ones, I like:
1)
https://adwords.google.com/select/KeywordToolExternal
Under "choose data to display", of course, I like "keyword search volume" but I absolutely love to display "search volume trends"... because it will show you whether your keyword is seasonal or a steady one.
2)
http://tools.seobook.com/general/keyword/
uses yahoo/overture data to estimate Google and MSN traffic
3) My Brain!
Although not commonly used as a keyword brainstorming tool, I think it's a great one!