Ad Server Beans are an open source ad server product written in Java using enterprise technologies. (GPL)

OASIS is a high-performance ad server built using Linux (RHEL4), Apache, PHP, and MySQL. It is designed to handle many hundreds of thousands of ad impressions per hour. (GPL)

OpenX Ad Server is a powerful open source platform that manages advertising for more than 150,000 websites in over 100 countries. (GPL)