Penumbra: Overture. The PC version hit back in March and provides a solid adventure game with more spooky atmosphere than you can shake a blood-covered piece of wood at. The game emphasizes interactive physics and physics-based puzzles, not combat, meaning you'll spend more time playing with the environment than stabbing baddies. Mac gamers can take heart, as Frictional is planning a Mac port of the game some time soon.
[Via Adventure Gamers]