Keywordspy has proven to be the best tool for harvesting keywords that I have seen. However it is not for finding those long tailed keywords for your site. Just a competition analyzer to see what they are bidding on. This helps you to possibly avoid those and look for niches they may have missed. Unless of course you are willing to go toe to toe with whoever your competiton is.
The free version is still useful but not as useful as the paid version.
Another VERY useful tool is
Keyword Elite. It seems a bit pricey but it is very effective. And like most of these products, you can get your money back if it doesn't work for you.