While taking a break from playing San Andreas to play catch-up with news and such, I stumbled onto an article at Slashdot about a homebrew version of GTA3 for the NES. Yes, you read that right; NES, the 8bit system.
Grand Theftendo is a remake of Grand Theft Auto III for the original Nintendo Entertainment System (NES). It is Grand Theft Auto III running on an 8 bit, 256×240 resolution, 2 bit colour x 2 bit palette, 1.79 Mhz system, written entirely in 6502 Assembly Language! It includes the entire Portland city!
The game is still in development, but from the info and screens available, it looks like it will be rather interesting. The developer, one Brian Provinciano, is in the proccess of recreating all of Portland using hand-drawn sprites and tile sets in a classic 8bit-style quasi-isometric top down yet slightly to the front-ish view. You can read more about it and check out some screenshots at GrandTheftendo.com.