OpenX
OpenX is an open source advertisement serving application. OpenX has previously been known as OpenAds and phpAdsNew, and OpenX forked from the phpAds application in 2000.
OpenX is a free, open source ad server that is trusted by more than 30,000 web publishers in over 100 countries around the world, making it the most used open source advertisment server. OpenX is a powerful ad serving solution that puts control over online advertising back in the hands of publishers.
-OpenX developer