Well, another game that created the gaming world as we know it today. The original Half-Life, as well as the two award winning expansions to it: Opposing Force, where you get to "become the bad guys" who hunt Gordon Freeman on his trip through Black Mesa, and Blue Shift, where you take the place of a lowly security guard of Black Mesa who later becomes a major character in the series (Barney).
This is only the single player versions of each game, but superbly easy to run on even the most modest system nowadays.
Enjoy the awesome storyline that created one of the most popular and well known franchises of our time. (and come on, this is where the whole crowbar thing started too )