Check out SpyFu (spyfoo.com). SpyFu allows you to type in a keyword and see how many Google Adword advertisers are on that particular keyword. It also gives you a breakdown of the following:

Cost/Click Range Cost/Day Range A listing of the current adword ads for that keyword Other keywords the competition purchased

The free service is limited to the Top 10 or 20 result set in most cases. Monthly subscriptions are $59.95.

www.spyfu.com