Portal was originally released as a standalone title on Steam and also in Valve's compendium The Orange Box, with few expecting that it would be anything more than a well-made, short-lived puzzle game that couldn't possibly live up to Valve's legacy with the Half-Life games. The game's singular premise revolves around the protagonist navigating a series of increasingly complex puzzles by way of a Portal Gun and an object called a Weighted Companion Cube. What made the game more than a competent-but-forgettable puzzler wasn't just its fiendish difficulty, but its vibrant personality. The malevolent computer wishing to send you to your death, GLaDOS, quickly became a fan favourite character, and the game's closing song, "Still Alive", has become a gaming anthem ever since. Many cake-related memes followed (relating to GLaDOS' constant promises of cake), as did a massively successful sequel, and the rest is history. Just don't count on a third game: we all know how much Valve hate the number 3...