Our sysadmins just installed Clamassassin to help manage spam and it works great. Clamassassin is a wrapper for Clam AntiVirus for use in procmail filters, much like SpamAssassin. ClamAV is a GPL anti-virus toolkit for UNIX designed for email scanning on mail gateways. It comes with freshclam, which updates the virus signature database several times a week.
For me, it’s filtering out more than 100 spam messages a day with no false positives that I’ve noticed. I’ve put this filter first, before the spamassassin filter, so some of these would have been caught anyway. But I’d guess it has reduced the amount of spam that makes it to my mail reading client by half.