Mari0

Mari0 (2012) is a fan-made Crossover between Super Mario Bros and Portal. You're Mario, and you have a portal gun. You can play through the classic levels from the original NES game (but with portals!), try and solve the Aperture Science-themed puzzles based on Portal and Portal 2, or even make your own levels. There's a Co-Op Multiplayer mode too--up to four players can play simultaneously, much like in New Super Mario Bros. Wii.

The game is available as a free download at its homepage on Stabyourself.net.

Tropes used in Mari0 include:

Co-Op Multiplayer: 4-player simultaneous, and each player gets his own portal gun.
Crossover
Dungeon Bypass: In addition to the ones present in the original game, the portal gun allows you to creatively get around some obstacles. Use portals for a perfect 5000 flag catch every time, portal behind Bowser to hit the axe, portal Bowser behind you to do the same, and many more.
Easy Level Trick: Only one player needs to reach the goal to complete the level in a co-op game. This can be used to bypass several companion cube puzzles.
Hoist by His Own Petard: Hammer Bros. can be killed by their own hammers sent through portals. Humorously, this used to extend to the hammers of other Hammer Bros. as well; a pair of them often thinned themselves out when the one in back started throwing. This was patched in version 1.4.
Knockback: An unlockable cheat has Mario ricochet behind himself whenever he shoots with the portal gun.
Konami Code Entering it at the title screen unlocks the cheats without having to complete the Super Mario Bros. levels first.
Level Editor
Minus World: Reachable.
Nostalgia Level: Up to Eleven. Literally the entirety of the original Super Mario Bros. is playable.
Retraux: 8-bit graphics, obviously, to duplicate the style of the original Super Mario Bros.
Shout-Out: The hats available include Rainbow Dash's mane (it's possible to perform the Sonic Rainboom with one of the cheats enabled!) and the eponymous striped top hat of The Cat in the Hat (which this TF2 hat is based on).
Virtual Paper Doll: Mario's sprite is customizable and can be recolored however you like, and you can even give him a variety of silly hats, mostly from Team Fortress 2.
Wreaking Havok