1001 Video GamesS.T.A.L.K.E.R.: Shadow of Chernobyl appears in the book 1001 Video Games You Must Play Before You Die by General Editor Tony Mott.
PDAAt some point of the game, you find one PDA called "Gordon's PDA". Some people say you find it in some scientist's corpse, but it seems to be more random than that. Reading it, the owner tells how he was first in Black Mesa, then in some Russian town and then here in Ukraine. This is clearly a reference to the Half-Life series and the possibility of that series' protagonist Gordon Freeman visiting the zone as an stalker.
The funny thing is how Freeman looks like a total rookie trying to survive in the Zone, to the point that he even has to trade his crowbar for a can of food. The bottom line comes when you realizes that you have just killed Gordon Freeman (which is why you have his PDA), who hasn't been able to survive to the Zone after all.