Google overture keyword tool.It's a good tool.I have used this tool.I use also seomoz pro,but the membership runs around 80$ a month for a bunch of tools.
Best free keyword research tool is Google Adwords keyword tool. It shows search volume and competition level of keywords that helps to determine keyword selection. If we are talking about paid keyword research tool then I'll vote for keyword spy.
IMO search engine are the best things to know about keyword count used by different people for different subjects so I always avoid any other tool if it is not provided by search engines itself.
According to me the best keyword research tool is Google Adwords keyword tool. It shows search volume and competition level of keywords that helps to determine keyword selection. In market other keyword tools are also available like SEO moz.