A friend of mine showed me Super Mario Bros. Crossover a few weeks ago, and it's awesome. It's as web based (Flash) game that is a complete recreation of the original Super Mario Bros. game. But it's not just that. Beyond playing as Mario, you can play through the whole game as characters like Link from Zelda, Samus from Metroid, Bill from Contra, and a few others. The characters play exactly like they played in their original games, but you are fighting Bowser's army of Goombas, Koopas and other evil bad guys. Very awesome!
In my spare time the last few weeks I've been playing the game through with all 6 characters. I guess I did it because I liked the original game so much. I recorded the last level of each character, and you can view these videos below. :)