S.T.A.L.K.E.R.: Lost Alpha. It's been out for about three months now. Technically it's more than just a mod for SoC - it's a free, standalone game. As I understand it, it's an overhaul from the ground up, depicting SoC "as it should've been" before final release. I'm only in the Cordon so far, but Dezowave has not only expanded on the textures and lighting, but also the maps, gameplay, NPCs, mutants, etc. And Zombies aren't just brain-dead S.T.A.L.K.E.R.s who lumber around with guns. They're actually the type of zombies you'd expect from most games that include them.