A modern MUD development kit.
The Agony Forge is a modern, web based MUD core. It is MIT licensed so you can use it for anything you want, free or commercial.
The Core is a library with all the basic elements you would need to create a web based MUD. You can't run it by itself, but you can fork the demo project and use that to get your own MUD started.
Please see the wiki for details about how to use the Agony Forge.