The developers of S.T.A.L.K.E.R. are working on a prequel called S.T.A.L.K.E.R.: Clear Sky:
Revealed in the latest edition of PC Zone magazine, Clear Sky is a prequel to Shadow of Chernobyl. Without giving away too many plot for fear of spouting spoilers, it fleshes out the origins of one of the main characters in the first game who, stepping into the boots of a stalker from a rival faction this time around, you're out to kill.
According to GSC, the Zone we stalked in Shadow of Chernobyl was one of relative peace - really? - but a year earlier when Clear Sky is set the environment is far more hostile. The developer describes the area as being "agitated", with anomalies "extremely active and the landscape is vastly different to what you've seen before."