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.
The S.T.A.L.K.E.R. 1935 Build is what I think to be the best of the builds. The game world feels more atmospheric and in-depth than the rest of the builds and the released games. Though old and bugged there are a lot of lessons and ideas to be had from this build. Hell even the "new" Call of Pripyat Blood-Sucker animations are in it. Sum it up: fun to play when patched with community fixes and just good.
Better sounds than in SoC, CS or CoP.
Only few levels looks good.
But it's just like a pre-alpha demo anyway :)
P.S.: It's a good build to take some resources from and put to SoC. The combination of the cut features and of the finished game is very good.