Xbox Emulation
One of the best features of a modded xbox is the sheer number of emulators ported to it. Couple this with the fact that these work out of the box on the console (no tweaking to get it right for your hardware as everyone has same hardware) and well designed interfaces to run the games and you are onto a good thing.
This means that you can play all those classic games from the dawn of computing. You can also play some quite recent stuff on the Nintendo 64 and Playstation.