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.
List of Doom source ports
The "Doom" engine's source code was released to the public on December 23, 1997. Although "Doom" was originally created for MS-DOS, the original source release was for the subsequent Linux version, due to the use of a proprietary sound library in the DOS version. The original purpose of source ports was cross-platform compatibility, but shortly after the release of the "Doom" source code, programmers were correcting old, unaddressed "Doom" bugs and deficiencies in their own source ports, and later on introducing their own modifications to enhance game features and alter gameplay.