Croquet uses edge-network technology to deliver the fastest and most seamless multiuser experience on the web. So this one is interesting to me as I’ve built things with my own version of this concept in the past. Essentially, they have WebSocket endpoints that you connect to and they just generically distribute your messages to all of the connected clients. No app-specific server side logic. You handle that on each client. Well, they handle it, with a library that they use to synchronize and probably deal with connection management, leader election and other distributed concerns. However it works, with 5,000 sessions and 1 billion messages (yes, billion with a ‘B’) per month, their free tier is worth checking out.