Just found out on the Romanian Linux Users Group list that BitDefender published a tool for fighting spam. It's not open source, but it's free (as in beer) and you can hook it into your favourite MTA!
Get it from here.
ToolsShow the IP address
Random password generator
ProjectsRepos - My RPM repositories for CentOS/(RH)EL
Linux LiveCDs - My CentOS based remixes
OpenVM.EU - Linux templates & appliances for Cloudstack