OpenX is an open source advertising banner system, and is the most-used free banner system on the Internet. Openads was previously known as OpenAds and phpAdsNew, which forked from the phpAds project in 2000.
From the Author
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. OpenX is a powerful ad serving solution that puts control over online advertising back in the hands of publishers.
Authors website: http://www.openx.org/