This season marks the 25th anniversary of Super Mario Bros. What better what to say "congrats" than with a fan-made Super Mario game that puts Super Mario Bros 1, 2, 3 and Super Mario World in a blender. Drink it up!
Available as a free download, the game also features a two-player co-op mode, a level editor and a battle mode. It's got 50 levels and five playable characters. Did I mention it's free? Well, it's free.
Super Mario Bros. X - Mario Fan Games [SuperMarioBrothers.org Thanks, Mark!]