spamassasin is a perl script which I've been using with good success to clear out spam. I have it setup to filter all my incoming mail, then hand-off to slocal.

Eg. /usr/local/bin/spamassassin -P | /usr/lib/nmh/slocal -user $1

My maildelivery looks for the special header added by spamassassin, X-Spam-Flag. The default rules catch about 80% of my spam, which I dump in my spam folder just in case I want to double-check the results. There are some very complex configuration rules one can build that result in each message being scored as potential spam. Users can extend the rules, and apply new scores for existing rules.

http://spamassassin.org/