PlayCanvas is an open source WebGL game engine and in-browser editor for building stunning HTML5 games, AR/VR experiences, and 3D visualizations.

GDevelop is a free, fast, no-code, AI-assisted game engine for creating 2D, 3D, and multiplayer games, targeting indies and studios.

icculus.org hosts a large collection of open-source software projects, primarily focusing on Linux ports, game engines, and development utilities.