Portal 2

Portal 2 is the sequel to Portal, the puzzle-platform game originally released as part of The Orange Box. It features significant upgrades across multiple aspects of the game, including its engine, graphics, physics, and gameplay systems.

It also features a larger single-player campaign, new characters, a co-op mode and an in-game level editor.

Its development cycle was particularly complex, going through multiple different storylines and design choices, especially BTS (Behind The Scenes), resulting in a substantial amount of development material and unused content.