Penumbra is the only adventure game in which you play an ostrich.
More specifically, you play a terrified but not terribly sane mathematician descending into the depths of an abandoned mineshaft-turned-military-base. And unlike most adventure games, this one is all about the atmosphere rather than the puzzles.
Although the gameplay is flat and the puzzles are simple, the ambiance is spectacular. The graphics, sounds, level design -- it's all very solid and frightening. This is one of those rare games that will put you in a cold sweat even if you play it on a laptop at a picnic.