Quake 3's code is open source but it relies on proprietary assets that you must get from buying the game, but they could make a freely-accessible web version of

Quake 3's code is open source but it relies on proprietary assets that you must get from buying the game, but they could make a freely-accessible web version of OpenArena (it's a free open source game that uses Quake 3's code and game mechanics with free assets).