Using SpamAssassin on Mac OS X
See http://www.stupidfool.org/docs/sa.html
Or http://niklas.sosdata.se/index_man_macosxcommunigate.php for using CommuniGate Pro on a Mac OS X 10.3 machine.
Tenon's Post.Office comes with a SpamAssassin module http://www.tenon.com/products/post_office (the 10 user version is free). Scripts and tips for using SpamAssassin and Post.Office are at: http://www.euregio.net/joe/postoffice/
The Post.Office Wiki has information about advanced SpamAssassin setups: http://dbmscan.com:3455/poss/58