The popular one used to be -
http://inventory.overture.com/
But it's almost always down anymore. I use wordze myself, which is paid, but I consider it a good thing. Makes the database a bit less accessible and more competitive.
I have to say though, having used paid Wordtracker for about a year as well - in my experience, these tools are inaccurate at best and sometimes give you really screwed up results. Better than nothing, but I take their predictions with a grain of salt.