Tilemap Town is a multiplayer game/platform where you can build maps with your friends and roleplay and chat in the worlds you have created. It takes inspiration from BYOND "building games" that used to be popular long ago, as well as from MUCKs and other text-based platforms.
Currently you can walk around and chat, and place/remove tiles on an existing map, or make your own map if you register. You can use your own 16x16 (or 32x32) player avatar or use a premade one, and in the future there will be scripting and custom tilesets. Want to help? Get in touch! There's also a small Discord server for Tilemap Town and other projects I work on.
The server is written in Python, making things easy to modify and extend. This is one of the really great benefits from starting over and leaving codebases from the 90s behind. The database is SQLite rather than being a custom solution, making it easy to access the database with other programs or explore it with a database browser.
Want to give it a try? Browse to the HTML5 client and login with a blank username and password to join as a guest to see what other people have built. It'll also work offline, if you just want to mess around on your own! Though you can't currently save anything you make offline. There's also a very basic client for the Nintendo 3DS.
I'm hoping to build a lighthearted and whimsical space where people can drop in and be in-character here! I think it would be a lot of fun to treat this world in-universe as being some sort of digital world that people can explore and easily build on. Of course if you're not interested in roleplaying you can just use Tilemap Town to chat and build.