In my fight against email spam, I have tried my hosting company’s spamassassin, gmail, but some spam still gets through.
I’ve now tried spambayes, and I find it works well. http://spambayes.sourceforge.net/
The main problem I had was getting my email client to talk to spambayes, and then gmail (spambayes doesn’t support gmail-level pop security (yet?).
I found a great workaround by following: http://www.engelassociates.net/email.html (and using stunnel).
Although gmail seems to catch the vast majority of my spam, spambayes adds a nice extra layer, for the spam that slips past gmail.