Stoked Rider is a snowboarding game with realistic physics (using Ageia's PhysX engine)--including fluid simulation and avalanches. You roam over a huge space--64 square kilometers of procedurally-created Alaskan moutains. No ski slopes, in other words--just you and the mountain.
Mouse motions are used to steer, with the WASD keys to lean, providing excellent control of your board. (Controls can be mapped to keys if you prefer, and game controllers are supported.)
You can board anywhere in the game-space--using either a helicopter to land anywhere you choose, or hiking to a starting spot.
All in all, a truly impressive effort by a small team.
(Oh, and what's "alien" doing in the title? Instead of playing a human, you can play an "alien" who has a jetpack--allowing some outrageous stunts. A little odd, but fun.)
Stoked Rider can be played in English, German, French, Spanish and Italian.