Been running SpamAssassin on the mail server for quite some time.
The following .cf files have reduced the amount of spam by a noticeable bit.

http://www.stearns.org/sa-blacklist/sa-blacklist.current
http://mywebpages.comcast.net/mkettler/sa/antidrug.cf
http://www.merchantsoverseas.com/wwwroot/gorilla/bigevil.cf

Throw them in your /etc/mail/spamassassin/ directory and restart spamd. Oh yeah, make sure and : mv sa-blacklist.current 50blacklist.cf.
Make sure you keep them up to date, that way you will be blocking all the new stuff..