Cube is an open source multiplayer and singleplayer first person shooter game.
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.
Most of the engine design is targeted at reaching feature richness through simplicity of structure and brute force, rather than finely tuned complexity.