Quake mod Nexuiz (pronounced "nexus") is so heavily modified that it's practically it's own game. It has been for years, since 2005, with a modified Quake Engine, completely overhauled weapons, but the same fast-paced gameplay. Someone at THQ must have been fond of the PC mod, because not only is it making a big comeback, it's coming back stacked.
Nexuiz, the new one that is, is a multiplayer-only XBLA title that looks and feels completely different from the original. It's not quite as fast, due mostly to the slower speed of its CryEngine 3 tech (more delicious screenshots here), but several key improvements have been made to overhaul the previously simplistic gameplay and give it more life.