What are the best couch/local co-op PC games?

D:OS2 features a Game Master mode, which lets you create custom multiplayer campaigns for up to 4 players, with the 5th player assuming the role of a Game Master.

When creating a campaign, you can use any asset within the game, such as characters, items, etc, and import new, user-made assets as well. You can even create new quests, levels, and alter any of the existing assets, allowing you to go as far as creating a sci-fi campaign if you have the patience.

Additionally, because there is a game master, you can create various role-playing events not possible in a normal campaign. For example, you can designate one of the playable characters as a kleptomaniac, so whenever they're near an object that can be stolen, you'll have to roll a dice to determine if that character resists the urge to steal or not.
On top of that, the game master can control NPCs, add quests, and make rule adjustments on the fly.

There's enough creative freedom to create any campaign/story you want, keeping you interested in D:OS2 long after you've finished the main campaign. Even if you don't want to create a campaign on your own, you can just download a campaign made by another user.