Spamassassin is soooo leet.
if you're running debian, try "apt-get install spamassassin procmail razor"
this installs it with support for vipul's razor, which is a distributed spam signature database.
workbench users can add:
to their .procmailrc file and it will just "work".
I haven't installed razor on workbench yet, I'm not sure whether I'll bother or not, because it will be alot of trouble.
(I love the logo, it has a copious amount of ninjas in it. w00t! )