It's commercial and proprietary software but it has an open-source "cousin" called Pure Data.
Both applications were created by Miller Puckette and his creations inspired a lot of more or less similar systems.
One worth noting is vvvv. It doesn't have musical programming roots and uses some unique and really powerful concepts to make graphics programming easier. http://vvvv.org/documentation/of-spreads-and-slices
Also it's a very pragmatic system which is built and maintained by people who do really cool and sometimes complex projects. http://www.meso.net/vvvv