I am currently using POPFile Bayesian filter (open source, Perl, works as POP proxy) with very good results. Dead simple, too.

I run an SMTP gateway, on a corporate level. I'll need something that will intercept the inbound SMTP connections, spam check with various tests (rbl, bayesian, whatever), then hand off to our main gateway. Ideally, something that would allow the users to train their profile with their own spam, with little interference from me.

-jk