In addition to the Overture keyword tool, there is one suggestion tool made by Google (has not graduated yet, however) which, if used right, yields great results... it can be found at
http://labs.google.com/sets. Again, it is a very helpful tool if it is used right.
If you would like more exact numbers for popular keywords, you should go with the Overture keyword tool or a service such as WordTracker. However, both of those show really skewed results, but do give you a good starting point.
Good luck!
Marc