Doom (1993): Shareware, Deathmatch and Mods Changed PC Gaming
id Software gave away the first episode and let players build their own levels.
On December 10, 1993, id Software released Doom. Led by programmer John Carmack and designer John Romero, it was fast, violent and technically ahead of anything else on PC.
The shareware gamble
id released the first episode free as shareware, copied across bulletin boards, university networks and magazine discs. Players who wanted more bought the rest. The approach put Doom on millions of PCs.
Deathmatch
Doom supported networked multiplayer, and its player-versus-player mode, deathmatch, gave the world a new word and an early form of competitive PC gaming.
Built to be modded
Doom kept its levels and assets in WAD files separate from the engine, so fans could create their own maps and total conversions. That community never stopped: Doom has since been ported to almost anything with a screen.
Legacy
For years, first-person shooters were simply called "Doom clones". The series continues today, and Doom's mix of free access and player creativity predicted modern free-to-play and user-generated content.
Reference: Wikipedia, Doom (1993) and id Software.