Using SpamAssassin on Mac OS X
Installing SpamAssassin: Benjamin Trott of MovableType describes the process of installing SpamAssassin on OS X. It assumes that you are using a POP mail server (it could be tweaked to work IMAP, but if you're using IMAP, the popread script won't work). It also assumes that you are using a mail client that can read from mailbox-format files.
Using Open Source Tools to Filter Email on Mac OS X Server: Apple steps you through the installation and setup of some widely-used tools: Perl-based Amasvisd-new, Spam Assassin, and ClamAV. There are other, similar open-source tools available, but completing this exercise shows you one way to keep your mail server clean of viruses; you can try other solutions, as well.
Using Stalker CommuniGate: on a Mac OS X 10.3 machine.
Tenon's Post.Office is a commercial package which comes with a SpamAssassin module (the 10 user version is free). Scripts and tips for using SpamAssassin and Post.Office. The Post.Office Wiki has information about advanced SpamAssassin setups.
AFP548.com have a great tutorial on setting up SpamAssassin in conjunction with Amavis and Clam-AV
See also: CommercialMacOSX