Ask HN: Is there a spam filtering web service?

After reading Resig's post about Google Groups, I was wondering if there exists a web service that specializes in filtering out spam.

I would imagine I could create a new filter, send training data to a filter, and send data to a filter and have it return the likelihood of it being spam.

Anybody have first hand experience using a web service like this? If so, has it met your expectations?