The basic concept of Proun is that you're racing along a pipe and avoiding obstacles and trying to get a high score. Gameplay-wise, it's a source of fun that will likely suck hours away from your week once you dig into it. But it's the visuals that really set it apart from anything before it. The name comes from the art of El Lissitzky, who would often use the made-up word "Proun" as a sort of place-holder for artworks (It also helps the games art is clearly influenced by Lissitzky).
The game comes loaded with five tracks, four different speeds, and a set of mod-tools for anyone looking to make their own levels. You can also play with up to four people via split screen or race against your old times with a ghost mode -- in short, it's basically a full game available for however much you think it's worth.
Control-wise, all you need to do is move horizontally along the pipe avoiding objects. You pick up speed based on how often you're missing objects. It's incredibly simple, but because of that, it's also remarkably addictive.
Give the game a shot for free and if you suddenly realize you've accidently dumped several hours into it, consider heading back and donating some cash.