The entire STALKER series is on sale today. There are few bargains better than the one of CoP, which is (IMHO) the best of the series:

S.T.A.L.K.E.R.: Shadow of Chernobyl - $4.99
S.T.A.L.K.E.R.: Call of Pripyat - $3.74
S.T.A.L.K.E.R.: Clear Sky - $2.49

If you play FPS and don't have these games, this is a very, very good deal.