Grit Game Engine - The Grit Game Engine is an MIT-Licensed Open Source Game Engine that is highly scriptable via Lua.
Reality Factory - Reality Factory is a free suite of game development tools for building 3D games.
Lumix Engine - Open source 3D game designer and engine written in C++.
CraftStudio - CraftStudio lets you craft games from scratch in real-time over the Internet.
MOS - Lightweight game engine.
Limon Engine - Limon is a multi platform 3D game engine mainly focusing on first person games.