Portal

Portal was created by Valve and reached players in 2007 as part of The Orange Box bundle. It turned the puzzle genre on its head: using a gun that fires a pair of linked portals, the player solves spatial puzzles where entering at one point means instantly exiting at another. The game's distinctive atmosphere came from the artificial intelligence GLaDOS, whose cold, darkly comic commentary guided players through the test chambers of the Aperture Science facility. One of the game's most iconic elements was the so-called Companion Cube, an ordinary crate presented as the player's only companion, which later became one of the series' most recognizable symbols.

The first game's success — which quickly turned it into a cult favorite, helped along by the end-credits song Still Alive — led to the release of Portal 2 in 2011. The sequel expanded the puzzle mechanics with new elements such as light bridges and colored gels that change surface properties, introduced more prominent characters including the robot Wheatley and former Aperture Science head Cave Johnson, and added a two-player cooperative campaign. Portal 2 is now widely regarded as the series' high point and one of the best-received sequels in video game history.

Both games came out on PC and on PlayStation 3 and Xbox 360, with Portal 2 also reaching Mac and Linux, helping it find an even wider audience. The series is still held up as a model of how a simple game mechanic — the portal — can be developed into intricate puzzles and strong storytelling, and its influence is visible in many later puzzle games.