Plot: Thinking with Time Machine is a single-player mod for Portal 2 developed by Stridemann, and released by SignHead Studio. It was initially released to Steam on April 18, 2014, for Microsoft Windows, OS X, and Linux systems, available freely to users who own Portal 2 on Steam. Wikipedia
Plot: Aperture Tag: The Paint Gun Testing Initiative is a 2014 first-person puzzle-platform video game developed by the Aperture Tag Team. Originally made as a mod of Valve's Portal series, it was released on July 15, 2014. Wikipedia
Plot: Portal Stories: Mel is a single-player mod for Portal 2 developed by Prism Studios. The mod was released on June 25, 2015, for Microsoft Windows, OS X and Linux systems, available freely to users who own Portal 2 on Steam. Wikipedia
Plot: Weekday Warrior is a mod for the video game Half-Life 2. It was developed by students at The Guildhall at SMU and released in its final state on 7 June 2006. Weekday Warrior received the Best Mod and best Best Singleplayer FPS Mod awards in the 2007 Independent Games Festival. Wikipedia
Plot: Narbacular Drop is an environmental puzzle video game developed by Nuclear Monkey Software. It was released for free online in 2005 for Microsoft Windows. It was the senior game project of students attending DigiPen Institute of Technology. Wikipedia
Plot: They Hunger is a single player horror mod for Half-Life developed by Black Widow Games. It is divided into three chapters, which were distributed on the internet over a span of three years. Current distributions of the game include all three chapters.