Frictional Games has released a beta demo of their upcoming horror-adventure game Penumbra: Overture for Linux. Discussion and troubleshooting for the beta can be found in this forum thread.
The demo is for Episode 1; Penumbra: Overture focuses on story, immersion, and puzzles with a first-person viewpoint. There are also preview videos at their media page. Frictional’s Edward Rudd also posted the following regarding the retail version:
Currently, however, for episode 1 the linux and mac versions will be separate from the Windows versions, and be available as download only. For Episode 2 we will hopefully have a unified CD with all three OSes on the CD.