OpenXRay, an enhanced game engine for S.T.A.L.K.E.R. Call of Pripyat has shown off progress towards running it on Linux.
While they say it's open source and they've thrown it up under a BSD-style license, I would hesitate to say it actually is. The reason being, it's based on leaked source code that isn't in any way approved of by GSC Game World.
Anyway, legal grey area aside they showed it off under Linux a few days ago:
Since the game is really quite old now, eight years in fact, it would be good if the rights holders would officially open it up. I'm not one to be overzealous about open source, but considering how long this leaked code has been out along with the age of the game itself, it would be a nice thing to do for the community to keep the game alive for many years to come. You still need to buy the game for the assets (I assume, it's not overly clear), so it's not as if GSC loses money from it.
Hat tip to velemas.