vvvv is a toolkit for real time video synthesis. It is designed to facilitate the handling of large media environments with physical interfaces, real-time motion graphics, audio and video that can interact with many users simultaneously.Check out the website, and free windows download: Realtime video synthesis - VVVV
I have also discovered Processing which has a mac version.
Processing is an open source programming language and environment for people who want to program images, animation, and interactions Link
All discovered via - 2063music.de which has some great free music to download!