While travelling recently, I decided to try Pure Data, the real-time graphical programming environment for audio, video, and graphical processing. I first heard about PD at an I/O Media jam in Toronto. Here is a picture of my first patch, a recursive reberb (click to expand):
Probably the best way to learn about PD is to read the free online book The Theory and Technique of Electronic Music. I'm slowly making my way through it.