VRChat - Create and play in virtual worlds with others
Castle Game Engine - An open-source 3D/2D game engine for ObjectPascal.
Second Life - Second Life is a virtual reality platform where individuals interact in a virtual world. The software was developed in 2003 by Linden Labs. More than one million people now regularly use the software.
Open 3D Engine - Open 3D Engine is an open-source 3D game development engine in real-time.
Vircadia - Vircadia is an open-source and decentralized real-time VR creation platform for university, enterprise, social, and OpenSim users.
Godot Engine - Feature-packed 2D and 3D open source game engine.