One of the reasons I switched to using Google's Gmail long ago was it's awesome spam filters. Previously, I tried a lot of solutions. About the only thing that's as good as Google's spam filters is the 1-2 combination of the Akisment and Spam Karma 2 plugins on Wordpress for blog comments.

This entertaining little promotional video that Google put together explains how Google's spam filter's work. Obviously, they aren't sharing big, corporate secrets here. The most important piece of the spam filtering equation is human, i.e. the users of Gmail.