Newer greylisting daemon for Postfix
for postfix, frankensteined from various bits of code to make this only require *one* instance running (not several). uses DBD::SQLite, a free local disk based SQL engine (CPAN will install all you need, no daemons needed). This is what I’m currently using as it is lower weight on the server.
Btw, sqlite rocks. It is worth looking at. You may not have a use for it today, but you may in the future. It is free, bundleable, requires no server, default now in PHP; and easy to install for perl (just install DBD::SQLite, done!).