Comment SPAM Wiper is a plugin for WordPress by intermod that checks your comments against the CSW web service to see if they are SPAM or HAM. If a SPAM comment is detected it is automatically moved to SPAM folder. If a SPAM comment is not detected and you manually mark it as SPAM, Comment SPAM Wiper will learn and in the future it will detect it as SPAM. CSW learns and improves as it goes.
You will need a free API Key (http://www.spamwipe.com/signup.html) to use it.