Reality Factory - Reality Factory is a free suite of game development tools for building 3D games.
VRChat - Create and play in virtual worlds with others
CraftStudio - CraftStudio lets you craft games from scratch in real-time over the Internet.
Spatial - Collaborate from anywhere in AR
Limon Engine - Limon is a multi platform 3D game engine mainly focusing on first person games.
Gather Town - Spatial video-chat worlds for work and play