If you like roguelikes, this is the game for you. If you don't like roguelikes, what are you, some kind of pussy?

An atmospheric title set in the area around Chernobyl in the near future, the first S.T.A.L.K.E.R. feels somewhat like Fallout, but quite different. I would suggest looking into either this game or Call of Pripyat (Clear Sky is just most of this game played with a different character and some minor updates) to anyone who likes "post-apocalypse" gameplay.

Borderlands has a little age, but that doesn't diminish its value any; its affordability lends itself well to 4-player shenanigans. You and up to three friends are pitted against a plethora of foes using a randomly-generated assortment of weaponry and items.
While the game has a "story", its merit lies in its ability to experience the entire thing with your friends. A large variety of side-quests and up to 3 playthroughs on a single character (with the level of enemies scaling each time) mean you could invest a lot of time into this game, even on a single "run".
However, Borderlands does have some issues. Some people I've played with complain of random (but eventual) crashes, and far too often the "loot chest" system decides it wants to give one character type all the gear (for instance, insisting that the Hunter needs more sniper rifles). Barring that, it is a classic for anyone with three friends and way too much time on their hands, and its GOTY award is well-deserved.