Random Play
For a good few years during and after University I enjoyed experimenting with generative art and music (I had a
couple of generative remixes released on Hydrogen Dukebox before starting
Type).
I dipped back into that world today and I'm really quite surprised at how easy it is to do things like this:

I used
Structure Synth and
Sunflow (both open source) to do this, with a little post-processing in Photoshop.
There are a bunch of other tools I'm looking to get back into and/or try out:
-
Processing - pretty much the winning generative art tool. Requires Java. I did some stuff with this for the Big Picture a few years back.
-
Processing.js - a javascript version of Processing for doing things like Algorithm Ink in a browser (not IE, though).
-
Nodebox - a Python generative environment.
For some great examples of what you can do, there's
Flickr,
openprocessing.org and
Generator.X, which shows what this stuff looks like as an output from a 3D printer.