Actions in the present state will alter the future, though manipulation of the future situation will not impact the present, allowing, for example, for one to bring a weighted cube set in the present from its position in the future, back through the time portal, and use that version of the cube in the present.

Portal Reloaded - A mod atop Portal 2 that introduces the ability to place a third portal that sends the player-characters or objects through between the present and a future state of the same test chamber.Portal: Prelude - An unofficial prequel to Portal, set before the activation of GLaDOS when scientists used to watch over the test chambers and instruct the test subjects.The mod won ModDB 2010 Mod of the Year awards for Best Original Art, Best Singleplayer Mod and Player's Choice Mod of the Year categories, eventually winning 2nd place at the latter. Nightmare House 2 - A horror-themed modification.Minerva - An episodic single-player mod based in the Half-Life 2 universe, using storytelling inspired by classic games such as Marathon and System Shock.The player hears the voices of Christopher and a doctor as they try to work out what is wrong with Christopher. In Korsakovia, the player travels through the delusions of Christopher, a man suffering from Korsakoff's Syndrome. It was developed by The Chinese Room and released September 19, 2009. Korsakovia - A single-player mod for Half-Life 2.The game acts as a side story to the movie where the player takes the role of the park's game warden, Robert Muldoon. Jurassic Life - A modification based on the first Jurassic Park film.Flipside - A side-scrolling platform game that allows the player to 'flip' the game world around to its backside, altering the means a player can traverse a level.Dear Esther - An experimental 'ghost story' created as a research project at the University of Portsmouth initially released as a free modification in 2008, a longer commercial version was developed and released in 2012.Coastline to Atmosphere - A mod set in the Half-Life 2 universe, following Gordon Freeman after the events of Half-Life 2.Black Mesa - A recreation of Half-Life using the more advanced Source engine.Instead of a portal gun, the player solves puzzles using a mix of these gels that they can spray onto surfaces with a tool they carry. Aperture Tag - A modification based on Portal 2 that recreates the essence of the game Tag: The Power of Paint, the inspiration for the various gels used in Portal 2.

Portal 2 aperture tag