Space Engine is a free (but not open source) program that allows you to fly through vast reaches of the universe. Along the way, you'll see some pretty amazing vistas and probably want to take screenshots of them. It incorporates a good amount of real-world data about the solar system, exoplanets and the universe in general with procedural generation of everything we don't know. [more inside]
2x0ng (and its predecessor, Xong) is a hybrid roguelikes that has the appearance, sound and mechanics of Atari age games. Levels are randomly generated, and the rules are up to you to discover. Here is a video review for your elucidation. Available for PC, Mac and Linux.
Behold the Moebius Gear. Includes a description of the entire process from modeling to fabrication for your reading pleasure.
Pixel City is a procedurally generated cityscape by Shamus Young. Procedurally generated graphics have a long history of producing attractive results with extremely small amounts of code, like Elevated, which was generated by just 4K of code, automatically generated video game content (also, Spore) or the generation of realistic water flows. Note the last demo reel may have been the test for a new film mentioned previously.
rand()% is an automated net radio station streaming real-time generative music. All audio is generated by algorithmic software applications and programs written by sound artists and programmers.
Substrate: one of the more striking uses of Processing I've seen so far. And quite urban-like, no? via Computing for Emergent Architecture.