If You've Ever Played Super Mario, You Need to See This
Super Mario Bros. X is a massive project that blends elements from Super Mario 1, 2, 3 and World. It has many power ups, such as the Hammer Suit, Tanooki Suit, Kuribo’s shoe, The Billy Gun, and many old and new Yoshis. You can also play the game with a friend in the 2 player co-op mode, where the screen seamlessly splits and combines as the players separate and rejoin:
SMBX is most notable for its extensive editor that allows you to create almost any kind of level that you can imagine. The real time editor lets you edit the level while playing it! You can also create your own episode using either the SMB3 or SMW styled world map, or you can create a Mario 64 style hub level and have the players collect stars to advance.
Download the game here!
Honorable mentions to Super Mario Crossover which brings characters like Mega Man and Link (and their abilities) to Mario.