I have had reasonable luck lately with SpamNet by Cloudmark, which filters most incoming spam for a reasonable monthly fee. Unfortunately, it limits the mail clients you can use - I don't believe it supports anything but Outlook and Outlook Express - and has spotty performance with catch-all email addresses. (That is, if you have a "catch-all" address, and are targeted by random email address generators, a lot of spam will get through. If you have defined a set of addresses, with the rest of your incoming email being dispatched to /dev/null, it usually works pretty well.)