SpamAssassin (SA) is a neat Email filter that tags incoming mails it thinks are spam. Its aggressiveness can be configured, and it comes with a decent default level. It's written in Perl and is licensed under the same license as Perl itself.
You can use SA in many places:
SpamAssassin doesn't just have to use local static rules. It can run many tests on the Internet:
See our SpamAssassinNotes page for help using SA.