They don't get super extensive, but romhacking.net offers a lot of patches for games that change a lot of how they work. Mostly re-translations and whatnot as far as the Final Fantasy games are concerned. But I have seen some people completely rework a couple of the games. I think our own black_orb has a hack for FF1 where you play as the shopkeepers rather than the warriors of light. But I'm not positive he has it hosted on that site. It's just a good place to start if you're looking for hacks of the older games