Active Spam Killer
Active Spam Killer takes an "Active" approach in solving the problem: Everytime an email is received, a message is sent back to the sender asking for confirmation. If that sender does not confirm the message, it remains queued for delivery. If the sender confirms, the message is delivered and removed from the queue.
BLOWBACK is based on a very simple idea: require a person sending me email to do something by hand in order to get their message delivered to me. When somebody sends me an email, BLOWBACK saves it on my machine, and sends back an automatic response email requiring the person to click a link (or reply to the message) in order to get their original message "released" to me.
Greylite is an implementation of a modified greylisting technology for fighting spam on mail servers running qmail. It does not require qmail to be patched, but is simply plugged in to the top of the qmail chain, thus working smoothly with already-patched installations. It is self-contained because it uses SQLite, and is easy to setup and maintain, small, and fast.
Pivot-Blacklist is a comment-spam blocking system for the Pivot blog system. It utilizes the MT-Blacklist data file from Jay Allen.
Spamikaze is an automated spam blocklist system, using spamtrap and other data to automatically build a DNSBL. Spamikaze is meant to make it easy for anybody to run a DNSBL.
surblhost checks if a hostname is blacklisted by surbl.org for being referenced in spam email. SURBL compiles their lists from several sources, including spamcop, spamassassin, phising data sources, and so on. Many spam filters use SURBL for detecting spam.
Traveler is a spam-resistant E-mail system that uses a random number generator to drive a "pronounceable password" generator.