Currently available in Beta on Steam for you to test out, the Cooperative Update for Portal Reloaded sounds like a lot of fun. For those who haven't played before: it's a free, community made modification for Portal 2. Building on the original concepts in the game by giving you a third portal allowing you to travel between different timelines.
The Cooperative Update is planned to arrive sometime in the Summer bringing with it 20 new puzzles, which you can play either online or via split-screen. It also gives new music, new menu backgrounds, new subtitle options and various upgrades to the single-player time travel logical to solve some bugs.
"The Cooperative Testing Initiative is sent back to the past to retrieve some very important data that has been lost to time. One of you is in charge of shooting regular portals while the other takes care of placing time portals. This creates some very interesting gameplay mechanics, as both players rely on their partner to travel through space and time respectively."
Find it free on Steam but you need to own Portal 2 to play.