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.
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.