Web site: cubeengine.com
Platform: Linux, OS X, Windows, BSD, UNIX
License: zlib license
First release: 2004
Cube – an open source multiplayer and singleplayer first person shooter game built on an entirely new and very unconventional engine. Cube is a landscape-style engine that pretends to be an indoor FPS engine, which combines very high precision dynamic occlusion culling with a form of geometric mipmapping on the whole world for dynamic LOD for configurable fps & graphic detail on most machines. Uses OpenGL & SDL.
Allows in-engine editing of geometry in full 3D (you fly around the map, point / drag stuff to select it / modify it), which can even be done simultaneously with others in multiplayer (a first!). Has simplistic but effective fine grain vertex lighting that looks like lightmapping and can do dynamic lights & shadows. Doesn’t need any kind of map precompilation, even lighting is done on the fly. Has very simplistic quad-tree world structure that can do slopes (heightfields with caps) and slants, water, does decent collision detection & physics, has client/server networking that goes a long way in giving a lag-free game experience, and features a Doom/Quake-style singleplayer (2 game modes, savegames) and multiplayer (12 game modes, master server / server browser, demo recording) game with some uncompromising brutal oldskool gameplay.
Cube 2: Sauerbraten is a free multiplayer & singleplayer first person shooter, the successor of the Cube FPS.
Much like the original Cube, the aim of this game is fun, old school deathmatch gameplay and also to allow map/geometry editing to be done cooperatively in-game.
The engine supporting the game is entirely original in code & design, and its code is Open Source (ZLIB license, read the docs for more on how you can use the engine).