Portal™ is a new single player game from Valve. Set in the mysterious Aperture Science Laboratories, Portal has been called one of the most innovative new games on the horizon and will offer gamers hours of unique gameplay.
I just started playing it, but it seems pretty neat. The best way I can describe it is that it's a single player puzzle game where you have to learn how to "escape" from a room and advance to the next level. Each successive level adds a new twist/challenge. Heck, it's free.