Godot Engine - Feature-packed 2D and 3D open source game engine.
Castle Game Engine - An open-source 3D/2D game engine for ObjectPascal.
Unreal Engine - Unreal Engine 4 is a suite of integrated tools for game developers to design and build games, simulations, and visualizations.
Open 3D Engine - Open 3D Engine is an open-source 3D game development engine in real-time.
BYOND - BYOND is the premier community for making and playing online multiplayer games.
WaveEngine - The definitive free cross-platform mobile engine to unleash your games using C#.