You're referring to the infamous "Half-Life 2: Platformer"!

The file relies on the user's desire to access the Half-Life 2 platform without authorization or for free. The term "platform" implies a necessary component for the game to run, lowering the victim's suspicion regarding the file's legitimacy.

Before the Steam Pipe update (which transitioned the engine to the .vpk system), Valve used the .gcf (Game Cache File) format. However, early builds and specialized tools were often packaged in .rar or .zip archives for easier manual deployment among developers or beta testers.