Xash3D FWGS Engine - Custom Gold Source engine rewritten from scratch.
Noa - Experimental voxel game engine.
nphysics - 2 and 3-dimensional rigid body physics engine for Rust.
Piston - Modular game engine written in Rust.
GDevelop - Open-source, cross-platform game engine designed to be used by everyone.
Corona - Simple to learn and use 2D game engine developed by Corona Labs.
VCMI - Open-source engine for Heroes of Might and Magic III.
Hex Engine - Modern 2D game engine for the browser.
RaZ - Modern & multiplatform 3D engine in C++17.
Flex Engine - Personal game engine.
bgfx - Cross-platform, graphics API agnostic, "Bring Your Own Engine/Framework" style rendering library.