I agree 100%.
Keywordspy is the best tool to find keywords for a given product or market.
Technically it is not a 'keyword tool' but rather a competition analyzer.

Keyword Spy gives you the keywords that advertisers actually use to bid on in Adwords. Also it tells you all the keywords a site ranks for organically in Google!

However, you need to clean up these keyword lists after downloading them.
Some popular sites have 5000 keywords or more - many of them being very similar or even the same.

Also it is pretty expensive.
But for me it is worth the money.