Id Software's Quake is the game that popularised online deathmatches. This first game in the Quake series is inspired by Gothic-style environments, medieval settings and Satanic imagery (similar to id's predecessor, Doom). It also has been the starting point and an influence on user mods, machinimas and speedruns.
Thanks to Trent Reznor, founder/leading singer of Nine Inch Nails, his music and sound design/effects for the game added to the creepiness factor of the game, other members of the band contributed to the soundtrack as well. Though in order to get the soundtrack for the Steam version, you need the Ultimate Quake Patch.
Quake is also the game that the original Team Fortress was based on by John Cook and Robin Walker (who are now working in Valve).
Its singleplayer campaign consists of four episodes of eight levels each (including a secret level). You play a protaganist only known as Ranger sent into a portal to defeat an enemy codenamed Quake. As you collect weapons/items, they are carried over to the next level. The higher level you reach, the more challenging and intense it becomes than the last.