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
The idea of making "Doom" easily modifiable was primarily backed by Carmack, a well-known supporter of copyleft and the hacker ideal of people sharing and building upon each other's work, and by John Romero, who had hacked games in his youth and wanted to allow other gamers to do the same. Not everybody in the id Software crew was happy with this development, however; some, including Jay Wilbur and Kevin Cloud, objected due to legal concerns and in the belief that it would not be of any benefit to the company's business.