Supercollider 3 is a language and synthesis system that is both free and beautiful. The language is object oriented, based on smalltalk with a C syntax. The supercollider synthesis server runs separately, and indeed can be controlled by other languages such as haskell and scheme. It runs on Mac OS X, Linux and Windows.
If you want to try it out, Julian Rohrhuber and friends will be giving an introductory workshop at the festival.
SuperCollider can be turned to all kinds of music, but here’s a video of what happens when lots of supercollider enthusiasts get together: