Old Mario - Allows you to walk slower, avoiding certain enemies! You can also whack enemies with a star rod, like the one Kamella has, but smaller. Side effects are Alchzemers , which makes it so that you forget what level you were just in, and it wears of after you beat a level.
Spin Mario - Pressing A allows you to do a spin jump. This is a really high jump, and it has pretty nice air time. Also, if you hold B while running, it allows you to spin really quick and go into any enemies.
Birdo Mario - Turns Mario pink and allows him to shoot eggs.
Inferno Mario - Turns Mario into full lava. He can throw giant Fire Balls.