Odamex is a free, cross-platform, modern client-server Doom engine
Retro game engine for creating games like it's 1997
Simple Minecraft-inspired program using Python and Pyglet
3D FPS Digital Paintball 3 Multiplayer Online Game
Dungeon Master is a RPG/RTS developed on the BYOND engine.
Open source, moddable game engine for the "Polygon Love 2" game
Game development toolkit
Misfit Cubed is a mod of Misfit3D that includes an MD5 exporter