Pixel City is a procedurally-generated city by Shamus Young. For the non-coders out there, this essentially means that based on a certain set of rules, this 3-D city is generated dynamically each time the program runs. Here, the video that shows the Young's process will make it more clear:
Check out the very detailed 10-part explanation for more on how Pixel City was built. Hopefully more comes out of it than just a screensaver. If it does become a screensaver though, I'd gladly use it.