Boodler: a programmable soundscape tool created by Andrew Plotkin.
Boodler is a tool for creating soundscapes — continuous, infinitely varying streams of sound. Boodler is designed to run in the background on a computer, maintaining whatever sound environment you desire.
Boodler is extensible, customizable, and modular. Each soundscape is a small piece of Python code — typically less than a page. A soundscape can incorporate other soundscapes; it can combine other soundscapes, switch between them, fade them in and out. This package comes with many example soundscapes. You can use these, modify them, combine them to arbitrary levels of complexity, or write your own.
Sounds pretty nifty and it apparently works under MacOS X with a little work.