Castle Game Engine - An open-source 3D/2D game engine for ObjectPascal.
Scratch - Scratch is the programming language & online community where young people create stories, games, & animations.
Open 3D Engine - Open 3D Engine is an open-source 3D game development engine in real-time.
Blockly - Blockly is a library for building visual programming editors.
WaveEngine - The definitive free cross-platform mobile engine to unleash your games using C#.
EduBlocks - Making the Transition from Scratch to Python easier.