Metro 2033 is more of a Call of Duty linear shooter, but you should know only the Call of Duty guys make that sort of thing like it should be. It's also a real strain on most computers, kind of competing with Crysis in terms of visuals. Near its ending, the difficulty gets crazy-stupid.
Red Orchestra is a kinda frustrating shooter at times, but it is nice to go back to one-shot rifles, and fun at times.
But STALKER is the one that sets itself apart with its adventuring. Skip out on Clear Sky, as it's the most broken, least interesting, and is just a prequel to the main storyline that doesn't really tell you anything. Shadow of Chernobyl and Call of Pripyat are often sold together, and the 3rd is a direct sequel.