Portal Reloaded is a free, community made modification for Portal 2. The mod builds on the concepts of the main game by allowing you to place a third portal, which enables traveling between two different timelines. Thinking in four dimensions is vital to solving 25 brand new mind-bending puzzles.
A free DLC for Portal 2 owners on Steam. The addition of a time portal really pushes the boundaries of the puzzle design and brings some fresh interactions that'll leave you scratching your head. It really feels like playing underwater 4D chess.