The listing, [Steam] S.T.A.L.K.E.R. Clear Sky -- Dystopian Shooter FPS has ended.
Found this Steam key just for you Listia guys.This game is not without its own idiosyncrasies, but is a good and gorgeous game nevertheless.
more info:
http://store.steampowered.com/app/20510/
me:
http://steamcommunity.com/id/OOO67VH/
tl;dr:
Steam free game software is at http://www.steampowered.com/
When Steam is installed, under the GAMES menu, select "Activate a game on Steam".
After two screens, you will be prompted to enter a key.
Keys will be registered into your account, like owning a virtual game disc.