I can't believe I just came across this game: Super Mario Bros Crossover. It lets you play classic Super Mario Bros as other NES characters like Link (from original Legend of Zelda), Megaman, Samus Aran (from Metroid), the dudes from Contra, Ryu (from Ninja Gaiden), and the tank from Master Blaster. Pretty sweet for some 80s nostalgia. And it's a flash game, so you don't have to install anything to play it. It has the ability to save your game, edit the map style (use tiles from different Super Mario Bros games) etc...
http://supermariobroscrossover.com/reso ... ssover.84/