We used Clickforensics (Now Adometry). They are one of the more well known companies dealing with click fraud. However, they only detect suspicious activity, then sends over the list of invalid clicks to Google. It's then up to Google to decide whether this list is accurate and how much one should be compensated for.
At the end of the day we never made a profit using their service because we were paying a couple of grand/mo while only receiving a fraction of that from Google. Google claims they do compensate you for invalid clicks and make automatic adjustments (sometimes as high as 10% off) to your bill. My advice is, don't bother. Unless you are spending hundreds of thousands every month, you won't see any value.