This is build 1935 of S.T.A.L.K.E.R. Shadow of Chernobyl, formally called S.T.A.L.K.E.R. Oblivion Lost. This was made by GSC Game World before they remade it and it was finally released in 2007. This was released by them after requests from fans of the game. This was uploaded by Audio Ghost, so that you all can enjoy the old version, which includes more places, vehicles, and other stuff that did not make it into the final released game.
Enjoy Stalkers :-)