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.
For example, get used to the guy in the screenshot. He stalks through the mine, attacking you whenever he sees you. I don't know what he eats besides you, but he does look pretty hungry. And diseased.
You're supposed to hide from him. But if you look at him while you're hiding, you panic and jump to your feet. So in order to stay hidden, you have to cower in a corner and stare at the wall until he's gone. Adds a lot of atmosphere, because I've never played an ostrich before.
They say the game is to show off their engine and their physics, but the truth is that the game is very well designed behind that. The smooth, subtle physics really do add a level of immersion that most games can't approach, and they aren't so focused on physics that it ruins the game. It's a nice balance.
The game isn't perfect, and it's a bit pricey... but if you're looking for a cold sweat, I haven't seen any other downloadable games pull it off so well.