S.T.A.L.K.E.R.: Shadow of Chernobyl is US$3.79 This weekend on Steam.

In 1986, the world's worst nuclear disaster occurred at the Chernobyl power station. Soviet authorities established a 30km 'Exclusion Zone' around this nuclear wasteland, but in 2006 a second explosion rocked the stricken reactor, obliterating all living things and causing the Zone's boundaries to ripple outwards.

A great deal if you have never played the game.  Source.