The developers of S.T.A.L.K.E.R., released an old, early version of their game to the public.
"'xrCore' build 1935, Oct 18 2004."
I tested it out a little, Seems to work pretty well on my crappy PC. XD
From what i understand, This build has some unused levels that didn't make it into the final version. The game is mostly in Russian but there is already some mods out that change it to English.