ActiveWorlds!

OpenSimulator is an open source multi-platform, multi-user 3D application server. It can be used to create a virtual environment (or world) which can be accessed through a variety of clients, on multiple protocols. It also has an optional facility (the Hypergrid) to allow users to visit other OpenSimulator installations across the web from their 'home' OpenSimulator installation. In this way, it is the basis of a nascent distributed Metaverse.

A 3D internet chatroom application for chatting with friends and meeting new people

Join the worlds largest virtual hangout where you can meet and make friends. Design your own rooms, collect cool furniture, throw parties and so much more! Create your FREE Habbo today!

Finally, a community-based virtual world filled with actual games and entertainment made by people who love creating games.

Build multiuser 3d scenes using html-like tags and javascript. SceneVR scenes are viewed using a browser-powered viewer (powered by webGL). Chrome, firefox and IE are supported. The server is powered by node.js and runs on Windows, Mac OS and Linux.

AltspaceVR is a social VR platform for meeting people from around the world, playing interactive games, and attending free live events. Choose from over 28 activities or attend live events with comedians, DJs, authors, celebrities, and more.

Vicarious is a social platform that lets you share and discover stories through an immersive experience accessible in 2D and 3D in virtual reality.

Smeet is a virtual 3D World, where you can create your Avatar and your own Room. Start now and meet new friends, flirt, organize your party, share pictures and videos!

vSide is an immersive, interconnected virtual city environment where you can hang out, listen to music, chat, dance, and make friends.