Released: N/A (Unreleased prototype), Windows
The Half-Life 2 prototype is a prototype of Half-Life 2 that was leaked in Fall 2003, less than a year before Half-Life 2 was released but a few days after the game missed its initial release date. Thanks to poor security on Valve's end, a hacker was able to remotely enter their hard drives and steal development data. While Valve eventually cut the connection, someone who knew the hacker got ahold of the prototype before the hacker was arrested. With the prototype's release, Half-Life fans learned that Valve was nowhere close to being done, despite it being leaked a few days after the game was supposed to be released. Oops.
The prototype not only contains resources of what would become the final game, it also contains many resources from earlier revisions such as scrapped weapons, level textures that are different in style from the ones used in the final game, removed props, characters that never appeared in the final game, and early textures for characters that did appear. It's not just a prototype; it's also something of an archive of the game's development.