While I enjoy the heck out of single and competitive multiplayer games, it's hard to match the fun of a well-produced co-op experience in my opinion. There's something particularly engrossing about tackling in-game obstacles with a coordinated effort. Unfortunately, few titles step up to the plate and those that do generally take a halfhearted swing, providing mediocre secondary campaigns and stale survival modes -- assuming you can even work through the connectivity issues that still plague many games.
I've had the best time with indie offerings such as Trine and Shank lately, likely because they're cheaper so I have lower expectations and they're often more conducive to local co-op, eliminating network problems from the equation. I preordered Torchlight II recently and I'm hoping Runic won't disappoint. Portal 2, both Left 4 Dead titles, Rayman Origins and Borderlands are some noteworthy retail releases with competent co-op elements, though the last uses GameSpy for online access which can be a headache.
Given the ongoing seasonal promotions at Steam and Green Man Gaming, it seems like a great time to list your favorite co-op PC games while there's still a chance for others to get them on sale. Naturally, you're also welcome to share your experience with any other co-op game, new or old, console or PC. As a side note, if you've been on the fence about grabbing Torchlight II, I have a spare copy of the original entry collecting dust. The first person without the game to message me on Steam can have it for free.