S.T.A.L.K.E.R.: The Lost Alpha
Basically it's a recreation of the first STALKER game but with redesigned levels and putting cut content back into the game. They've also really updated the game's engine with some absolutely amazing lighting effects. That said though it's not the most optimized game and runs like ass on my PC, so probably not something you want to try and max out.
Oh and it's free and doesn't require owning the original game.
S.T.A.L.K.E.R. - Lost Alpha mod - Mod DB