An article in the New York Times technology section talks about fighting spam.  I recently started using SpamAssassin on my personal POP server.  So far, I'm quite pleased with the results.  The program uses rules to score mail as SPAM.  Right now, I'm just redirecting it to a different mailbox and reading it to make sure it works and doesn't throw out too many things I'd really like to see.  So far, nothing I can't live without.