The first edition of Portal (2007) changed how players think about virtual environments. As you freely explore a mysterious laboratory, you encounter physical puzzles and challenges by opening portals to maneuver objects through space.

Portal 2 (2011) contains more content, new characters and a larger set of test chambers. The helper in this game is once again GLaDOS. The two player mode lets you act and think cooperatively. Other content includes a set of editing tools. Created by the Seattle-based team at Valve Software (who also runs the Steam online store).