CloudMark SpamNet.

http://www.cloudmark.com/products/spamnet/

...uses a hash of the email to see if anyone else has reported it as spam. If so, it moves it to a spam folder. It's based on Vipul's Razor:

http://razor.sourceforge.net/

It's cut my spam intake by about 95% at work.

At home, I use SpamAssassin driven from my procmailrc file. See http://www.spamassassin.org/
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-- roger