This has got to be one of the most awesome things I've ever seen on the internet...
It isn't just some ****** flash game that attempts to recreate the original Mario Bros. with other characters plugged in... this is the real game, with characters from other classic Nintendo games that are programmed and behave exactly as they do in the games they come from (following the rules of Mario Bros. regarding mushroom power ups and so on.)
Characters include Ryu from Ninja Gaiden, Samus, Megaman, Simon (Castlvania), Link, the guy from Contra, and some others. It's really something else.