Super Mario Crossover
This is the entire game of Super Mario Bros, but now you can play as different characters. Each character plays just like they do in their original games with a few modifications to make the experience better. They can even grab mushrooms and stars for different effects. Between each level, it's possible to swap character, allowing you to try out each in a variety of different Mario settings.
Use the arrow keys to move your character.
Use Z and X keys for action.