As Clear Sky is a prequel to SoC, its far shorter, at around 10 hours. It's design is more linear in that you are transported to areas of the Zone rather than treking there yourself, and there are therefore lots of smaller maps coming together to make the Zone itself.
It's a top game though, still got that STALKER magic, especially at night time when its completely pitch black, and you hear gunfire followed by nashing of teeth. Time to run!
Oblivion Lost is a complete overhaul which aims to build on the atmosphere of SoC, and times it by ten. It turns the Zone into a brutal place, where you'll find yourself fighting bandits simply for vodka to prevent radiation. It's probably the best community mod I've ever played, and considering I've played most of those for Half Life 2, thats saying a lot.
In the words of the mod team, the aim is "Total free play, modified A-Life, reality enhancement, better AI, faction wars with cooperative game play."
You'll need to start a new game with it, otherwise it will corrupt. Whether it works with the 2009 mod I'm not sure, although I dont see why not.
Oblivion Lost.
Call of Pripyat completes the trilogy later this year, and the devs have said it is returning to the open world feel of SoC, whilst encorporating the graphics and action of Clear Sky.
