Your new favorite IP-infringing time-killer: Super Mario Crossover, which lets you tackle SMB as a range of 8-bit heroes: Mega Man, Link, Samus, Castlevania‘s Simon, Contra‘s Bill, or, for the unimaginative, Mario himself. More than just a cute idea, Exploding Rabbit have gone to some length to correctly tailor the specific physics and play styles of each guest star to a wicked degree.
For more retro mashups, see also: Rom Check Fail, by Captain Forever creator Farbs.