QuakeWorld, the godfather of online gaming. The birth of many online gaming services was due to QuakeWorld and most modern FPS (first person shooters) games are based on the original Quake engine.
QuakeWorld is the ultimate adrenalin rush, with netcode ahead of it's time, the FPS genre was pushed from a single player experience to a fully multiplayer online game which spawned a huge following of players.
With it's open structure, many modifications (known as mods) were created by the online community, some of them becoming as popular as the original game itself. Perhaps the 2 most famous are Team Fortress and Painkeep. New maps, weapons and objectives provided players with a change from the frantic world of deathmatch and tactics became a major part of the game.
Deathmatch is still the base of the large majority of FPS's but the team element is taking an increasingly important role.