Indie studio Stabyourself has just released Mari0 (that's a zero, not an 'O'), a remake of the original Super Mario Bros. with the teensy, tiny addition of a Portal gun. But the fun doesn't stop there! There's four-player co-op, free DLC, game modifiers, map packs, a level editor, and hats! So scoot on over to the link below and download the game for either PC, Mac, or Linux.
Be advised that a lot of users have been having trouble installing Mari0. The creators have this thread on their forum to help get your game up and running properly.