L4D is an amazing game and I’m sure they are working on HL2: EP3.
L4D is probably the first game I can think of that really tried to create the perfect online co-op experience, at the same time attempted to have incredible replayability, and reinvent the idea of characterization and story development in an only multiplayer game. I would consider it a total success, and hope other companies take some inspiration from it. Co-op like this is just so well done.