Pulling apart some of the game's textures reveals stunning similarities with, for example, water textures from Half-Life 2 and lighting textures from Doom 3. In the case of Doom, some of the texture filenames were even the same between the two games, with "lights_impflash.dds" present in STALKER, and referring to the Doom Imp monster.
STALKER seen running on borrowed code
Half-Life 2, Doom 3 Assets in S.T.A.L.K.E.R.? - Shacknews