Google's keyword tool works quite well. I was faithful to the overture keyword tool for years. Eventually it slowed right down to the point that it wasn't really usable. Then Yahoo eventually removed it a little after buying Overture.
You will see option for keyword optimization. In IBP you get a list of major search engines. Select search engines in which you want optimize. there is a tool for this.
It is not free Trial version does't have much. Perhaps nothing should be free when it's come to making money
Yahoo keyword tools is still available for some markets but for the US/main english ones you are out of luck.