Though the recent Retro City Rampage is a parody of older games, it is not, in fact, an 8-Bit game. Well, not until Vblank Entertainment decided they wanted to make it into a real 8-bit game.
Here they are, walking us through how they crunched the game down to 8-bit. It's all technical stuff, but not inaccessible (or uninteresting!) Kind of makes you wonder what other modern games can be crunched down like this, provided tweaks and ingenuity.