Cube is a 3D First Person Shooter that uses OpenGL and SDL.  It features:

  - Single- and multi-player gameplay
  - In-engine editing of geometry in full 3D (you fly around the map, point
    and drag stuff to select or modify it), which can even be done with
    multiple people at once
  - Simplistic, but effective fine grain vertex lighting that looks like
    lightmapping and can do dynamic lights and shadows
  - No need for any kind of map precompilation, even lighting is done on fly
  - Very simplistic quad-tree world structure that can do slopes (height-
    fields with caps) and slants, water
  - Decent collision detection and physics
  - Client/server networking that goes a long way in giving a lag-free game
    experience
  - Doom/Quake-style singleplayer and multiplayer game with some
    uncompromising brutal old-school gameplay

WWW: http://www.cubeengine.com/