S.T.A.L.K.E.R. - Lost Alpha is a free mod for S.T.A.L.K.E.R. Shadow of Chernobyl game. After finishing the story, you can go back and do some sidequests, which will only appear in the jobs list after the ending. The engine is now totally upgraded. This means, you don't need any SoC patches for LA, since now it became a standalone (but still free) game.