Following on from the humble indie bundle, if you liked Penumbra: Overture, why not buy the rest of the collection, and get 75% off with this code? The collection includes all three games:
Penumbra: Overture is the first game in the series, Penumbra: Black Plague is the second title and Penumbra: Requiem is an expansion for Black Plague.
Use code IDONATED to get at this price. Steam price currently £17.99 (77% discount!).
Available on Windows, Mac and Linux