Here’s a noise generator made in Processing which creates cloudy-looking noise which tiles! It’s extremely inefficient, but the algorithm looks really neat while it’s working!
Here’s a sample of its output, manually tiled (and scaled) to show that it’s seamless.
Check out my first foray into Processing: a 2.5D mosaic experiment.
Waaay faster as a standalone application (or run from within Processing).
One thing that I really like is the way you can see the waves reflecting off the walls. Sure, this isn’t exactly the most spectacular example of emergent behaviour, but it’s certainly not explicitly coded in. I think that’s pretty neat.