Technobloggie has a beta tool for estimating the number of searches a keyword gets on Google per month. It gives some good info as you can see below. Here is an example for the term "search engine optimization"
Searches a month: 18107
Max Bid: $3.53
Market Value: $63917.71
AdSense Effective CPM: $14.13 (Assumes a 20% revenue share from adSense, and a 2% CTR on ad units)
Listings on Google: 1490000
I have not done any kind of testing to see how this compares with the results of the other keyword tools out there, but it is looks like it might be useful. Check it out at www.technobloggie.com/googleSearchEstimates.php