UPDATE: Super Mario Bros. Crossover 3.0 has been released!

Super Mario Bros. Crossover is a fan game that recreates the original Super Mario Bros. and allows you to play it as characters from other games. Made by Jay Pavlina of Exploding Rabbit.

For optimal performance, play the game in the Firefox web browser. If the game runs too slow, turn the music quality down to low or turn the music off completely. If you experience a blank white screen, upgrade to the latest version of Flash. If the game appears cropped and you cannot see the whole screen, zoom your browser out to the default setting and reload the page.

Link to Official Site

Super Mario Bros. Crossover 3.0
(release date End of July – August 2013?)

…Also check out this commercial game Super Retro Squad…