SpamAssassin is a neat mail filter that tags incoming mails it thinks are spam. It can be set up to be a bit nastier about this, but the default is good enough.
You can use procmail(8)? to run SpamAssassin, or you might want to use MailScanner to run it on all your incoming mail. You might even want to plug VipulsRazor into it.
Its homepage can be found 
here.
SpamAssassin is written in Perl and is licensed under the same license as Perl itself.
(Note, this product will assassinate Spam, but will leave SPAM well alone!)
