Ranvier is a MUD game engine. Out of the box Ranvier is configured to use a telnet server and will work with any MUD client you've used to connect to other MUDs. It runs a websocket server simultaneously, meaning players can also connect to your game with a websocket client at the same time. However, Ranvier supports any transport server you want to use; changing the entire networking layer is as easy as adding a command. Ranvier also has a GUI builder available, Soma.