Nidhogg
/Chances are you haven't heard of this game but in a nutshell it is basically a 2 player 2D Prince of Persia wherein you fight over the priority of the screen and try to get to the opposite end. If I try to explain it now, this post might be as long as a review so instead I'll just show you videos of its gameplay.
From what I've heard, this can be played on a single keyboard so have fun yelling at your friends when you steal victory from their sweaty palms.
[youtube http://www.youtube.com/watch?v=Q2sZ4pV5vkY&w=480&h=390]
[youtube http://www.youtube.com/watch?v=VxPvFFagQD0&w=480&h=390]