We maintain in real-time a vast database of millions of IP addresses. We have recently decided to open our database to public to help fight Spam.
You can use our RBL for free. It will allow you to block up to 90% of the SPAM received by your servers.
Our RBL source server is: rbl.smtpcheck.net
It can be implemented using the rblsmtpd program. See: http://cr.yp.to/ucspi-tcp/rblsmtpd.html