This is a highly personal list, dealing mainly with the virtual worlds I would consider using for my classes. I’ve already taught or given presentations in some of them, and I continue to explore new ones. The information below is based on my own experiences and is not comprehensive, though I try to be as inclusive as possible.
The first 3D virtual world, created in 2003, SL has a large user base, and a vibrant artistic and social scene, although some areas are NSFW. It is accessed through a free client (Mac and Windows). Fees apply if you wish to have a private area reserved for your use. These areas can be switched between public and student-only access, but this does not prevent students from visiting other areas in the virtual world.
Many educators prefer to rent an area from an individual or group in SL, and there are educational collectives that make land use fairly inexpensive (see SL Groups).
OpenSim is an offspring of Second Life, with many interconnected grids running the open source Second Life software. Several free viewers can be downloaded, notably Firestorm and Singularity. Some grids are closed, others are open to the public. Some grids, like Kitely, offer setup and maintenance.
High Fidelity is a new company, running on completely different software; like Second Life, it can be accessed on a home computer using a free client, but it can also be used with the new virtual reality gear like Leap Motion and Oculus Rift. It is in beta testing now (September 2016). So far, content creation seems to be somewhat demanding, but a marketplace is developing, and hopefully educational tools will be available. HF’s attention to the audio environment make it a strong candidate for music education and immersive sites for student exploration.
Sansar is a new platform created by Linden Lab, the company that runs Second Life. Like High Fidelity, it can be used with a home computer, and also aims at attracting users of the new virtual reality headgear. Sansar is scheduled to open to the public in January of 2017.