I've been playing with POPFile, (
http://popfile.sourceforge.net ) an open-source naive bayes mail filtering proxy that runs on your PC. It's extremely easy to set up and use, and it works with multiple POP accounts. It doesn't delete any incoming mail on its own, but it will tag message headers according to the "buckets" it sorts the mail into (eg: "[spam]" and "[non-spam]" on my system); you can then use whatever filtering techniques you want based on the inserted header tag.
In any case, after receiving about 650 e-mails, it's now classifying them at about 96% accuracy; this should improve as the program learns more about my message flow.
Hope this helps.