Postfix greylisting policy service with auto-white-listing written in Perl with SQL database as storage backend. Greylisting stops 50 to 90 % junk mails (spam and virus) before they reach your Postfix server (saves BW, user time and CPU time).
I'm the main developer, of course I like it :-) Seriously, it gets the job done without getting in the way.
Does what it says on the tin. Generally requires very little in terms of management or server resources.