go and try google adword tool , it will help you and you can even know about advertiser keywords also .
Adwords keyword search is the most important...It actually shows you competition for keywords and search popularity...Wordtracker is good too...I tend to use both to nail a really good specific term, so use both...Use the Wordtracker free option as Mahendras states.
Check the number of sponsored links for a keyword,if sponsored links exists for 6 or 7 words then consider it as a high traffic keyword If you do not see any sponsored links then consider it as no competition keyword.