Multi-User Dungeons, A.K.A. MUD's, used to be more common than they are today. A MUD is a fully, or at least mostly, texted based game. Think of it as an ongoing, interactive work of fiction.
Some people new to MUD's are probably thinking, "You mean like Twitter?"
Uh, ... well, ... minus the 140 character limitation, ... kind of.
MUD servers are commonly in the high fantasy genre, but don't have to be. They can be on any topic ranging from sci-fi, spy vs spy, romance, and even Sims style games. These days, most people play massive multi-player, 3D games. But it is quite relaxing to sit back and figure out a riddle, or slay dragons in a completely text based environment with no worries about the dexterity of your thumbs or mouse-hand. To quote CoffeeMud's website, "a MUD can be the most addictive, rewarding leisure experience of your life, if you give it a chance. There are also numerous web sites and hundreds if not thousands of MUDs to play, each with their own special characteristics and twists to enhance the experience for all concerned."
If you want to create your own MUD, there are several options. However, all of them include running your own MUD server and probably making some changes to the source code. You will have to learn some programming or IT-type skills at the very least, if you want to make your MUD available to the general public.
Of the several options for MUD servers out there, you have MUD servers that are modified *nix OSes. Many MUD's are written in C++, C and other languages. You should pick a MUD server that is written in a language that you know or feel comfortable picking up. Knowing that Java is one of the most popular programming languages, I levitated instantly to CoffeeMud.
Before I get too much into the scary technical details, I should acknowledge that creating a MUD is solidly in the realm of geekdom, but you don't have to be an IT professional or Software Engineer to set up your own. Being a bit nerdy will work. As it turns out, CoffeeMud takes a lot of work out of the configuration and building of you MUD.
For those looking to get into the code quickly, CoffeeMud is easy to set up in Eclipse. I managed to get the code compiling in eclipse in about an afternoon. You might find having the sources and binaries in the same directories to be annoying if that is not your taste, but Eclipse handles it well.
CoffeeMud works with most databases, or with the build-in fake database. Mostly, CoffeeMud only uses the database when starting up, or when saving state. Other than that the need for a database is very minimal. You may ignore the database requirement if you want to and stick with the build-in fake database.
CoffeeMud comes with a web based MudGrinder application for creating your Multi-User Dungeon, but also has great command line tools available within the application from inside the game. If you create your own world inside of CoffeeMud, you will likely find yourself using a combination of the command line tool in the game and the HTML based UI for creating rooms, MOBs, items, and quests.
There is tons of documentation for CoffeeMud. You may find everything you need just in the docs. However, if that proves to be missing information, there is also a Yahoo support group. Also, I've found the author of CoffeeMud to be willing to answer question in the past. Many MUD's are lacking documentation and support, but CoffeeMud seems to have none of these issues.
CoffeeMud has many available races, classes,spells, skills and monsters ready to go. You can also add your own using in-game tools, the MudGrinder, or making changes to the Java codebase of the MUD. There are several sample quests to learn from hidden away in the code, also.
Along with being mature, the code is showing a few signs of its age. You will find that the provided web pages are not "responsive" in the HTML5 sense of the word and may have display problems on mobile devices. The on-webpage MUD connection is an applet, ... yes applets have been shunned by the industry and by browser makers. Also, the standard connection to the MUD is still a traditional telnet connection that would make any security conscious sysadmin cringe in pain and fear.
However, those are the only real concerns.
Another great feature of CoffeeMud is that it is distributed under an Apache License. That means that if you manage to pull off a level of success for your MUD that no one else in history has, you can legally make a profit off your MUD universe. (We can all legally have our professional-Dungeon-Master dreams.)
If you are the type that enjoys a good book as-much-as or more than a good movie, you should check out some of the more popular MUD's out there. And if you are really adventurous, you might want to download a copy of CoffeeMud and get it up and running on your own server, and begin creating your own world.