SpamCheetah is a network-level spam filtering solution built around OpenBSD greylisting, tarpit, and blacklisting. It comes with a Web interface and requires zero administration. It does not allow spam to even land in your network, thus saving you precious bandwidth. It does not have the 'false positives' problem of losing legitimate mail.
RetinaPost is a PHP class/WP plugin that makes CAPTCHA's clean and engaging using feeds or custom text. It is easy and intuitive for your Web site visitors. You can choose to use your feed or custom text to inform or guide the user to read more articles or to perform other actions. Use RetinaPost to stop spam and bots on forms, on comment posts, new account registrations, polls and surveys, and more.
pgl4rbl implements a Postfix policy server that mixes two widely used techniques: greylisting and RBL (DNS blacklists). The idea is that greylisting is applied only to SMTP clients that match a RBL; this means that normal clients are not delayed (such as with a normal greylisting implementation), and RBL false positives do not cause problems (like when outright blocking them at the SMTP level).
Synspam uses Netfilter NFQUEUE to catch the source IP address of any machine connecting to your mail server, running multiple tests against it (RBL check, regexp on the reverse name, etc.) before forwarding the connection to the MTA. In order to have as few false positives as possible, a scoring system is used. If the source IP address is believed to be a spammer IP address, the connection can be dropped. There is a dry run mode if you just want to test it, which is the default.