Unreal Tournament 1999, original intended to be nothing more than an expansion pack for the original Unreal which would simply be called "The bot pack", turned out to be one of the most influential multi-player shooters of all time. Toppling the giant powerhouse that was ID software in both technology and game-play and modding capabilities of ID's engines.
Unreal technology was a huge advancement over that of the Quake and Quake 2. (ID tech 2 and ID tech 3). It allowed for huge open worlds, coloured lighting, fog and volumetric effects and many other things that where a huge innovation at the time.
Unreal technology has powered at least hundreds of games since the late 1990's till today, and with the upcoming Unreal Technology 4 around the corner it looks like it will continue to be one of the most popular engines for some time to come.