Doom

John Carmack and John Romero's team at id Software uploaded Doom to a University of Wisconsin–Madison FTP server at midnight on December 10, 1993 — after 30 straight hours of testing. About 10,000 people tried to download the shareware's first episode simultaneously and crashed the network. The fast 2.5D engine was the hook, but the genius was architectural: level data lived in separate WAD files ("Where's All the Data?"), letting anyone with the right tools build their own levels. Thousands of user-made levels appeared within the first few years, seeding a modding culture that would outlive the game itself.

Multiplayer was added about a month before release. Romero coined the term "deathmatch" — killing another player became "fragging," a term that would stick in the FPS lexicon forever. Doom II dropped October 10, 1994 and became the United States' highest-selling software product that year, moving over 1.2 million copies within twelve months. By late 1995, Doom was estimated to be installed on more computers than Windows 95, and the shareware had been downloaded 20 million times by June 1996.

The gore and demonic imagery triggered moral panic. Germany restricted it as harmful to minors until 2011, and after the Columbine shooting in April 1999 — the shooters were known players — Doom sat at the center of the games-and-violence debate. A rumor persisted that one shooter had built a level of his school; he had made custom levels, but none based on the school. The source code was released in 1997 (GPL in 1999), ensuring endless ports and spawning the eternal meme: "Can it run Doom?" For half a decade, office networks and LAN parties had a Doom fragfest running in the corner.