ChucK is Princeton Universitys: Concurrent, On-the-fly Audio Programming Language.
"what is it? : ChucK is a new audio programming language for real-time synthesis, composition, and performance, which runs on MacOS X, Windows, and Linux. ChucK presents a new time-based concurrent programming model, which supports a more precise and fundamental level of expressiveness, as well as multiple, simultaneous, dynamic control rates, a precise and straightforward concurrent programming model, and the ability to add, remove, and modify code, on-the-fly, while the program is running, without stopping or restarting. It offers composers, researchers, and performers a powerful and flexible programming tool for building and experimenting with complex audio synthesis programs, and real-time interactive control.
You can download ChucK here. There is also another link to the Princeton Soundlab and all of the software created by the soundlab.
I haven't had a chance to download or play with it yet, but I imagine that it's not for the faint of heart. However, Princeton University has a new music ensemble called PLOrk dedicated to using ChucK as a basis for the performance.