Some notable open-source games and engines I'd like to see:

  NetHack/NetHack
  angband/angband
  crawl/crawl
  tmewett/BrogueCE
  wesnoth/wesnoth
  freeciv/freeciv
  godotengine/godot
  CleverRaven/Cataclysm-DDA
  OpenTTD/OpenTTD
  spring/spring
  0ad/0ad
  the-infocom-files/zork1
  s-macke/starflight-reverse
  keendreams/keen
  id-Software/wolf3d
  id-Software/DOOM
  id-Software/Quake
  id-Software/Quake-2
  id-Software/Quake-III-Arena
  id-Software/DOOM-3-BFG
  xu4-engine/u4
  exult/exult
  scummvm/scummvm