There are 4 repositories under virtual-worlds topic.
A list of synthetic dataset and tools for computer vision
A Multi-User Dungeon server in Haskell.
Lightweight client for connecting to Second Life and OpenSim based virtual worlds
Virtual Reality Engine - Interactive 3D Navigator over the Internet
Dolby.io Virtual Worlds plugin for Unreal Engine
WhiteCoreSim - Software to create your own virtual world
A Node.js Multi-User Dungeon (MUD) Framework
Dolby.io Virtual World plugin for Unity.
Pimak Worlds is a project about creating a 3D virtual world and stuff (discontinued, see https://github.com/7185/lemuria instead)
A virtual to physical world value system and vice versa
New worlds, many views
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External Tools Repository for WhiteCore
This repository is NOT production worthy nor useable
Lightweight client for connecting to Second Life and OpenSim based virtual worlds
Christmas Village on the Web with different quests and interactions.