This post was published more than a few years ago (on 2005-02-14) and may contain inaccurate technical information, outmoded thoughts, or cringe takes. Proceed at your own risk.
I've tested a few spam filters for my email over the last year, including the built-in one in Apple Mail, SpamFire and SpamSieve, and decided on the latter of the three as my final solution. SpamSieve does a great job, using Bayesian filtering to detect the probability of a message being spam, and it integrates well with Apple Mail, as well as several other email clients.
The programmer, Michael Tsai, was interviewed in Daring Fireball a while back, which is mighty interesting, and he also wrote the eminently useful DropDMG.