The best keyword search tool is your brain and the people who know the market best.
Stick to this simple formula
Keyword search tool = Use Google + Your Brain
Than comes, Google AdWords Keyword tool as a good adviser for your keyword curiosity and hunger
https://adwords.google.com/select/KeywordToolExternal
Do not hesitate to use and play with Google Suggestion keyword tool http://www.google.com/webhp?complete=1&hl=en, Google Trends http://www.google.com/trends and hot trends http://www.google.com/trends/hottrends that started in May 2007.
Now, you might ask me how should you use your brain? Hmmm tricky ah.
Also use and see:
http://hotsearches.aol.com/search/ AOL HOT SEARCHES
http://www.google.com/press/zeitgeist.html Google Zeitgeist
