A few years ago, a guy named Shamus Young coded something really cool: a procedurally generated city. Using just a few assets (a bit of texture, some general building models, a bit of code for cars) he makes and entire world appear. He explains it much better than I do in the video. You can download it here. It exists as a Windows screensaver file -- .scr, remember those? -- and is a whopping 127K.