It's called Super Mario Crossover and is hosted over at Newgrounds. Basically you can play through the original Super Mario Bros with Mario OR a selection of Simon Belmont, Megaman, Bill (from Contra), Samus and Link. Got your attention yet?

The coolest thing is each character has there individual games move-set as well as accompanying music. It really is quite awesome and it gives you an excuse to play through one of the best games of all time, so BONUS! Great job to the people (or in this case ONE GUY!) at Exploding Rabbit for making this gem!

http://www.newgrounds.com/portal/view/534416

P.S. you can play the thing with a Gamepad!

P.P.S. IT'S AWESOME, PLAY NOW