S.T.A.L.K.E.R.: Shadow of Chernobyl
The body of Gordon Freeman from the Half-Life series can be found in the Wild Territory area. Searching his body, it's possible to find his PDA. In it, he mentions both Black Mesa from the first Half-Life title, and "some strange town in Russia" (most likely being City 17 from Half-Life 2), and how he had to sell his crowbar for a can of meat. Next to him is a crate with a Desert Eagle called "Big Ben." This could ether be a reference to the famous clock tower of the same name in London, or, since it is near Gorden's body, a reference to Half-Life's creator, Gabe Newell, specifically his nickname, Gaben.
F.E.A.R.: Perseus Mandate
In Intreval 07, by using God-mode to get to the bottom of an elevator shaft, one can find "I suck at making maps" written on a wall.
The Witcher 2: Assassins of Kings
During the American President Barack Obama's tour of Europe in 2008, Polish Prime Minister Donald Tusk gave him several gifts in celebration of his visit to Poland, one of which was a copy of The Witcher 2: Assassins of Kings collector's edition.
Shantae and the Pirate's Curse
Once Shantae enters the "Tan Line Temple", the servants of the temple mistake her for the long-lost princess and dress her in skimpy royal garb (which is itself a variant of her belly dancer attire). Later, She comments that she's "dressed like a space princess". This is a reference to Star Wars: Return of the Jedi, and the costume Leia wears after being captured by Jabba the Hutt.
Metal Gear Rising: Revengeance
When rescuing a civilian, it's possible to catch up to them and slash them, resulting in their clothes being ripped off, leaving them in just their underwear. Raiden will then receive a codec call, scolding him. Calling people using the codec immediately afterwards will result in more negative comments being made.