Grit Game Engine - The Grit Game Engine is an MIT-Licensed Open Source Game Engine that is highly scriptable via Lua.
Castle Game Engine - An open-source 3D/2D game engine for ObjectPascal.
libtwo - All-purpose c++ app prototyping library, focused towards live graphical apps and games
Open 3D Engine - Open 3D Engine is an open-source 3D game development engine in real-time.
MOS - Lightweight game engine.
Godot Engine - Feature-packed 2D and 3D open source game engine.