Portal 2

Portal 2, released in 2011, draws from the award-winning formula of innovative gameplay, story, and music that earned the original Portal over 70 industry accolades and created a cult following. Portal 2 introduces many new characters, puzzle elements, test chambers, and environments. The game’s two-player cooperative mode features its own entirely separate campaign with a unique story, test chambers, and two new player characters.

Portal 2 brings some new technical things to Source. Most notably, improved shadows from, which the game uses extensively. Portal 2 was built off of a very early version of Dota 2, which came from Alien Swarm, so it inherits many of Alien Swarm's features (and sometimes bugs). Portal 2 has a fairly large amount of unused code and script files from Half-Life 2 and Portal in it, suggesting that the game could have been more connected to the Half-Life world at some point. Another possible piece of evidence is the entity, though it's probably just a leftover from Half-Life 2: Episode 3 development.