UO has a super robust eco system of private shards and server software. It’s kind of amazing. Pretty active development on things like ServUO or ModernUO servers. The client has been fully rewritten and actively developed with improvements on ClassicUO. All of these are open source as well.

For servers you have places that function entirely as different eras of UO like the Renaissance shard, or even entirely new content like what’s in Outlands.

There’s honestly a lot to be found out there and it’s really neat.