Today GSC Game World announced they're working on a true sequel to 2007's STALKER: Shadow of Chernobyl and it should be coming out in 2012.
They released Clear Sky which was a prequel to the events in SOC and then the follow up to all the stuff that happened in the first game Call of Pripyat came out after.
GSC is considering all three of those games to be part 1 in the STALKER story and now we'll finally see the true sequel to those games.
STALKER 2 will not use the X-Ray engine with GSC making an all new multiplayform engine for the new game.
I'm excited to see more STALKER. The series especially Shadow of Chernobyl is one of my favorites and I'm looking forward to exploring the zone once again.