Bundle Stars is currently offering S.T.A.L.K.E.R. Shadow of Chernobyl, S.T.A.L.K.E.R. Clear Sky, and S.T.A.L.K.E.R. Call of Pripyat for only $9.99 altogether. You can also grab each separately with the original costing $9.99 and the other two listed at $4.99.
If you don’t know what S.T.A.L.K.E.R. is, besides difficult to type when not using copy+paste, is a game that is excellent at atmosphere but a bit obtuse in actual gameplay. The closest comparison in premise and execution is the Metro series (Metro: 2033 and Metro: Last Light), although unlike Metro, S.T.A.L.K.E.R. is more about open environments for the player to explore.
The first S.T.A.L.K.E.R. released in 2007, then quickly followed by Clear Sky in 2008, and the final game, Call of Pripyat, in 2010. All were developed by GSC Game World which shut down in 2011 but reformed in 2014 and are currently developing Cossacks 3, a remake of their early real-time strategy title.