Doom (1993 video game)
The ability for others to create custom levels and otherwise modify the game using WAD files turned out to be a popular aspect of "Doom". Gaining the first large mod-making community, "Doom" affected the culture surrounding first-person shooters, and also the industry. Several future professional game designers started their careers making "Doom" WADs as a hobby, among them Tim Willits, who later became the lead designer at id Software.
Doom WAD
John Carmack, lead programmer at id Software, designed the "Doom" internals from the ground up to allow players to extend the game. For that reason, game data such as levels, graphics, sound effects, and music are stored separately from the game engine, in "WAD files". This allowed players to make their own data without making any modifications to the engine. According to "Doom"s initial design document, WAD stands for "Where's All the Data?".