STALKER? I thought it was one of those typical FPS shooters... it's a sandbox game is it? I'll have to see more about it, but which one would you recommend (since there are multiple of these)?
Alas, Stalker was poorly promoted by then-publisher THQ, who decided to sell it as a horror FPS. The game is about horror and FPS mechanics, but it is much more than that. Stalker: Shadow of Chernobyl is a sandbox game about exploring a strange land. While there is a main story to follow, you are not obligated to do so and can go off and do your own thing. In my opinion, Stalker is one of the few examples of a game that come close to the whole "gaming as art" argument as it feels like you're starring in a great sci-fi horror movie at times (if only Hollywood knew how to make movies as good as Stalker these days
).
There are two additions to Stalker: Clear Sky and Call of Pripyat. I haven't tried the former (I understand it is the weakest of the three), but I do own the latter and it seems like a nice follow-on. I would start with the original, though, as SoC establishes the core story and mechanics.
BTW: Stalker is based on the classic Russian sci-fi novella,
Roadside Picnic. I highly recommend it. While the game follows a different story (and setting, for that matter), the fundamentals are all there.