At home, I use Mozilla 1.3 to filter unwanted spam messages, but at work I've been using Cloudmark's SpamNet product for Microsoft Outlook. Both work really well, but SpamNet is now a paid service. I recently read an article in InfoWorld about an open source project called SpamBayes that uses a similar method that Mozilla uses to filter out spam. I am trying the Outlook plugin that they offer; so far seems to be fairly effective.