↑Mari0 is a ↑Super Mario Bros. plus ↑Portal mashup by ↑Stabyourself.net, which in turn “is a two-man studio formed in early 2011 aiming to create enjoyable experiences for anyone to easily get into and enjoy.” The two men are Maurice Guégan and Sašo Smolej, who already have published ↓Not Tetris 2.
    Mari0 is not a gamemod in the strict sense of the term, as the whole game consists of original code from the base up:

Two genre defining games from completely different eras: Nintendo’s Super Mario Bros. and Valve’s Portal. These two games managed to give Platformers and First-Person Puzzle Games a solid place in the video game world. But what if Nintendo teamed up with Valve and recreated the famous Mario game with Portal gun mechanics?
    A complete from scratch recreation of Super Mario Bros. with a focus on perfectly imitating the feel the 1985 classic gave us. Then give Mario a portal gun, add puzzle game mechanics from Portal and there you go. And if that wasn’t crazy enough, play simultaneously with as many friends as you can connect joypads to your PC, with everyone having their own Portal gun!

It ain’t released yet, but at ↑Maurice’s YouTube channel there’s a ton more of the above, and at ↑Stabyourself.net there are regular updates with screenshots, many of them animazed gifs of the newest achievements.
    All this reminds me a bit of “Dangerous Dave in Copyright Infringement” (1990), the legendary proof of concept by Carmack and Romero. I sincerely hope that Nintendo will be a good sport and see Mari0 as a parody, and not as a case for the copyright-lawyers.
    Just to round it up, here’s stabyourself’s text commenting their contact section: “Are you amazed at how awesome we are? Are you angry one of our games set your computer on fire? Are you Nintendo and want to send us a cease and desist letter?”