Valve really knows how to do co-op, eh? Let's add another Valve entry to the list: Portal 2. Not only did Valve put co-op into Portal 2, but it's entirely separate from the singleplayer campaign. You and a friend (or enemy) each control a robot, for a total of four portals at a time. Puzzles often require strategic use of all four portals, adding a layer of complexity to the proceedings as you try to coordinate with each other.
It's fun, though it'll also leave you screaming, "No, PUT THE DAMN PORTAL OVER THERE," and gritting your teeth at regular intervals.