Game mods are pretty neat. They can instantly grant players all sorts of new and exciting possibilities from new modes to character swaps. In the case of the latter, YouTube user Megamaniscrazy integrated the skins of Mega Man, X and Zero into Sonic Generations. It sounds like an odd pairing at first, but I think you'll agree the end result looks very cool.
Food for thought: is this the sort of game we could see if Capcom and Sega choose to take the Archie crossover one step further?
Source: Sonic Generations mod: MegaMan and Rayman (100th Video)