Opensimulator is the open source project (http://opensimulator.org/)

There are thousands and thousands of active regions and thousands of users (https://opensimworld.com/). It's not millions, but it s the best self-hosted virtual world ecosystem.

That's incredibly surprising that it doesn't have one. I was involved in the l2j and wow emulation communities back in the day, and there was just so much effort going into it.

It does have one:

http://opensimulator.org/wiki/Main_Page

Cool! Sorry, it's been 10 years since I last paid attention to them, so I have no idea where those communities have migrated to.