Collection: S.T.A.L.K.E.R.

The S.T.A.L.K.E.R. series was originally called Oblivion Lost, and was supposed to be a squad based action game.

Throughout the series, the in game energy drink item features the name of the first game, S.T.A.L.K.E.R.: Shadow of Chernobyl, on the can.

There is an unused human zombie enemy in the game files for all three games. It is thought they were meant to be the end result of zombified stalkers who have lost the ability to operate mechanical devices.
The PDA Encyclopedia also contains an unused entry for them;
"The Brain Scorcher strips its victim of mind and reason, leaving only an empty, mindless shell."

While not included in the games, all the necessary files are contained in the installed data, making them easy to mod back in.

The ingame Cossack vodka is a reference to the Cossack series by GSC games, the developers of the S.T.A.L.K.E.R. franchise. The artwork from the original Cossack game is used on the label, and the item's description reads "Made by GSC."