Godot is a powerful cross-platform game engine
Amateur 3D Game Engine
Vivid3D is a C++ IDE/3D engine, with python scripting.
Modern 3D engine and IDE written using C# and C++.
Asphyre is a cross-platform framework for Delphi and FreePascal.
Mahasona engine is an open source game engine.
DF modular backend
Multi-threaded 2D Renderer licensed under the Mozilla Public License
3D world editor for game development