Portal 2

Portal 2 is the sequel to the award winning and hugely successful Portal developed by Valve Corporation. It features a single-player story set after the events of Portal's story; and a brand-new Co-op game mode, featuring additional test chambers designed specifically for Co-operative play. Portal 2 was released April 19, 2011 on the Steam platform. Retail release in North America started on April 19, 2011; and later, Europe and Australia on April 21, 2011.

Development
Development on Portal 2 begins shortly after Portal was released. It originally was planned to not include any portals in the game, utilizing a new mechanic called "F-STOP". Portal 2 was also planned to be set in Aperture Science's earlier days, specifically during the 1950's era. When asked one of the members that worked on Portal 2 that the game wasn't going to include portals, and how would the game be a sequel to Portal, and more specifically, use the Portal title. He replied with, "We'll deal with it later.", hinting it was to focus on the game's mechanics first.

Portal 2 was also conceptualized as set after the events of Portal, and GLaDOS would test the new subject, named Mel, similarly to how it was with Chell. After playtesters were disappointed after GLaDOS didn't recognize the player as the one who killed her in the first game, Mel was rejected as a result.

Mel also appeared alongside Chell in the co-op mode of Portal 2 for a while before both being ultimately replaced by Atlas and P-body.

There was also going to be a fourth Gel planned for the game. Also inspired by "Tag: The Power of Paint", the fourth Gel was named Adhesion Gel. Bearing a purple texture, it originally allowed Chell to walk up walls. It was canceled due to playtesters became too disoriented and often had motion sickness while trying to pass a Test Chamber with Adhesion Gel. The coding (although it does not have any texture and bestows no effects on the surface it was painted on), is left in the game files.

Sixense
The Portal 2 Sixense MotionPack DLC is an exclusive DLC available to owners of the Razer Hydra. The DLC includes specific levels and mechanics that make use of the Hydra's motion detection functionality.

Peer Review
Peer Review is the first DLC released for Portal 2. Released on October 4th, 2011; this DLC introduces Challenge modes for both single-player and co-op, a leaderboards system, and a mini-hub area reserved for additional co-op tracks. This DLC adds the first of many additional courses to come with the introduction of the Art Therapy testing course.

Perpetual Testing Initiative
Perpetual Testing Initiative is a DLC released on May 8th, 2012; this DLC adds the anticipated in-game Puzzle Creator - fully integrated with the Steam Workshop, allowing the community to easily create, share, and play a wide range of Portal 2 puzzles.

The DLC also adds the stick figure "Bendy" as a playable test subject, to avoid interfering with the storyline by having Chell test the community chambers.

Portal 2 in schools
During a keynote presentation at the festival in 2011, Valve director Gabe Newell mentioned that the company was beginning to work with schools to build curricula around Portal 2, looking into "how to teach attraction, repulsion... [they] need to be clear on what their metrics are... what the outcome is, what they're optimizing for".