phpMyBlacklist: A project for creating and managing an email blacklist or RBL lists. The project includes steps for setting up the required zone in your name server, the required database, the web administration and a simple shell cronjob script.
IMAP4 server working on top of SQL database (currently PostgreSQL but Oracle may work too). It has some unique features like elimination of duplicate messages throughout the whole system, possibility of clustering or load balancing etc.