I currently administer 30 exchange servers at multiple companies. For the companies that want spam filtering we are using Barracuda spam filters, it is a 1U Spam appliance, and is pretty much plug and play, you tell it the IP of your mail server, and once you train it, it works wonderfully. I believe they do offer free evaluation units, and like I said the companies who I have them at are very happy with them. The company that makes it is Barracuda Networks, and a simple Google search for Barracuda spam filter pops up there URL.

Good luck with your search