A long time ago, Jeremy Elson ran a MUD at JHU called CircleMUD. That MUD no longer exists, so there isn't one specific MUD which is CircleMUD any more, despite what anyone else may tell you. Although he maintains the code and releases new versions periodically, he doesn't actually use the code to run a production MUD. So, if you want to see what CircleMUD is all about, the best way to get started quickly is to find someone who is using the Circle code to run their own MUD, and play it.
Some CircleMUDs adhere closely to the original, and others have been heavily modified by their administrators. Some have dozens of people playing at the same time, and others have only one or two. Try out some of the ones listed below and see which tickle your fancy.
Below is a list of some of the known sites that use CircleMUD, including a brief description of the MUD written by its administrators. There are many others that aren't listed here, some of which are included in various other MUD lists such as the very comprehensive MUD Connector.
If you would like your Circle-based MUD added to this list, please visit circlemud.net for our automated MUD registration page. The list below is updated nightly from the database. (Please limit your description to a maximum of 8 lines of 80 characters.) The page will check your responses but we reserve the right to edit your description to correct errors and we also reserve the right to remove your listing from this list if the site is down for an excessive period or if it is in violation of the license agreements (Both DikuMUD and CircleMUD apply).