I use a methodology called S25R [1] that involves regex filters in postfix. Super simple, no third party anything for those regex patterns. I reject the spammers and use iptables for the really bad spammer networks. In terms of external third party resources, I also use spamcop and uceprotect in postfix to do RBL checking.

[1] - http://www.gabacho-net.jp/en/anti-spam/anti-spam-system.html