Alternatives to Cube 2: Sauerbraten

Multiplayer code editor

First-person shooter, first in a trilogy

First-person shooter, second in a trilogy

2D and 3D game engine

Online multiplayer game platform

First-person shooter, third in a trilogy

Kart racing game

Frontend for emulators, game engines and media players (OpenGL graphics API)

Open-source, cross-platform game engine designed to be used by everyone

Real-time strategy game engine for Westwood games

First person shooter engine

Open-source continuation of Bungie's Marathon 2 game engine