Probably no S.T.A.L.K.E.R. fans in here but w/e, S.T.A.L.K.E.R. Lost Alpha is a Standalone mod that just got released. They've basically gone back and released what they feel Shadow of Chernobyl should have been.
All free to download and no original game required.
For those that don't know about S.T.A.L.K.E.R. it's a Survival Horror / FPS set after the Chernobyl Nuclear Plant Disaster. After the disaster, radiation is now at deadly levels all over what is now known as "The Zone", inside the zone animals have turned into aggresive mutated versions of their former selves. Also strange anomalies effecting the physics of the zone have been spontaniously appearing in random locations, stepping onto these anomalies could do anything from flinging you up into the air and tearing you apart to melting the flesh from your bones.
But these anomalies have also given birth to strange artifacts which scientists have discovered to posess properties which allow them to have effects such as healing and protection from other anomalies for the person holding them. This makes artifacts very valuable inside and outside the Zone. So bandits, thieves, scientists and nothing to lose jobless bums have come from all over seeking these artifacts to get rich outside the zone. Rumour has it that in the center of the zone is a very powerful artifact they call the Wish Granter, like the name suggests it will grant you your wishes, providing you can get to it alive.
tl;dr: everything in the zone wants to kill you. explore, get rich, find the wish granter, kill strelok.
now get out of here, stalker.