Spam Assassin

Spam Assassin is the world's biggest open source spam filtering project. It interfaces with most popular mail transfer agents and is a rule-based system that comes with thousands of spam filtering rules. It does a reasonably good job of filtering spam, especially if you have nothing else. It is written in Perl and is somewhat of a resource hog, so if you are going to run a lot of email through it, you're going to need a lot of computing horsepower to make it happen.