Rui CarmoHalf-Life is a Quake-based game (derived from the Quake I base source with added geometry and physics tweaks to resemble the Quake II engine).
It pioneered centralised CD-key validation for online FPS, and spawned uncounted ways of getting around such protection schemes. However, it was one of the first FPS where it became possible for game administrators to ban specific users regardless of online nicknames, IP addresses, etc. (one of the features I used to be most grateful for when I maintained a games site called accao.net).