I decide to post this list since I can't find a single list that contains all the tools I use (which isn't many):
Google AdWords Keyword Tool, Keyword Sandbox Tool Good tool but it's Google after all, so keep in mind that you and everyone else on the planet is using it.
MIVA Keyword Generator Note this is from the UK, so spellings might be non-USA
Overture Keywords Suggestion Tool Same problem here as with Google - any keywords you generate here will probably not be unique. Also somewhat of a pain, because it it alphabetizes the keyword varients it returns. So "cheap tennis racket" varients ceom out as "cheap racket tennis", "american express credit card" varients come out as "american card credit express" etc. Annoying
Search Engine Keyword Tracker & Keyword Ranking Tool This on is hand because it generates some Wordtracker words as well as the Overture ones. That's what I use it for. For some reason, the Overture lookup here sometimes seems to be faster than the actual Overture site.
7Search Keyword Suggestion Tool. Brings back 100 at a time. Good varients here.
The Miva and 7Search tools are pretty good, and probably not as widely used as the Google and Overture tools. I tend to use them for my main research and then augment with Google and Overture.
I'm pretty sure Microsoft has one too, but I can't find a link to it. Maybe you have to be an adCenter user to get access....