Proudly necroing this thread to say that System Shock 2 is on Steam and has co-op, and to everyone who even clicked on this thread after reading the title: if you haven't played that classic game, it's the same as never having played Half-Life, Deus Ex, or other foundations of the genre. You know you've meant to get around to it!

I can state without reservation that it is better than any game in the "spiritual successor" Bioshock series, all of which I own and enjoy, and, with a now-standard suite of high-resolution and game-enhancing patches and mods updating everything from enemy models to weapons balance (do a Google search for "GOG system shock 2 mods"), the game more than holds its own against many contemporary FPS with co-op. Little ones need lots of meat to grow up big and strong.