Heads Up! Ultimate Chicken Horse
As someone who considers trolling her friends the ultimate expression of love, Ultimate Chicken Horse appeals to me on a lot of levels. It’s platforming game built by the players. The goal is to build a level that you can beat while making it difficult enough that your opponent can’t. It’s both cooperative, since you have to work together to build the demo, and competitive.
You can download demos for Windows, Mac and Linux at the Ultimate Chicken Horse website.