Emulators and hacks

I've recently gotten back into emulators (old school stuff, Game boy SNES, Genesis) and have found an interesting subtype of ROM/ Essentially, they take the original game and modify the programming, often creating an entirely new game.

For instance, I have a hack of Super Metroid that lets you play as Dark Samus on a completely different world map and a hack of Chrono Trigger that is essentially a sequel.

If you're an emulator what interesting hacks have you found?

Hacks are hardly new.

However, there's a thread for it.