Many people search for the 'best keyword tool' but one thing I've found is that in reality you have to use all of them to get the best results. Even if each keyword tool you use gives you just one more iteration that you hadn't got on your list, then it's well worth it. In the keyword business, one single keyword can make you a fortune. So it's vital to use every keyword tool you can.

Especially make use of the free keyword tools. Google have an excellent keyword tool now. And Overture's, although very dated now, is still an excellent way to build up a beefy keyword list. Also look at Keywords Analyzer. I know I mention this tool a lot, but I use it every day and it's nothing short of a delight to use. It's the one paid tool I wouldn't want to be without.

You will find that various keyword mining tools do produce many similar variations. But it's up to you to hunt out the keywords that you don't have and make the most of each tools own idiosyncrasies.