Historically, to play a MUD, all one needed was a network connection and a telnet client. Telnet is a communication protocol much like HTTP (Hypertext Transfer Protocol), except instead of reaching out to a website using a web browser to bring up a web page, telnet allowed you via a command prompt to talk with and control a distant server. All in text.
Today's not so different, that is, most MUDS still host telnet ports that you can connect to, but players and programmers quickly figured out they needed something more that could store player credentials, edit messages, save connections to specific games, suppress MUD output and perform many dozens of other tasks that telnet alone could not do.
Thus the MUD client was born.
Many of today's MUDs host their own clients. For example, the most recent version of DikuMUD, which you can sign onto via Valhalla MUD, has a web client built right into it.
You can also download independent clients that will let you play on most any MUD that uses telnet. Some of the more popular clients include the following:
TinTin++ - Perhaps the oldest MUD client in existence, TinTin++ is a command-line client that runs on all three major operating systems—Windows, MacOS and Linux. It also runs on both major phone platforms, Android and iOS, although playing MUDs without a physical keyboard is neither easy nor a recommended experience.
You can read a brief history of TinTin++ here.
Mudlet - Another MUD client made for all operating systems, Mudlet is a graphical client that is in active development. As recently as February, 2023, the app developers offered a bounty program to fix some issues with their client.
ZMud - In its time, ZMud was perhaps the most popular of all MUD clients, so much so that its creator later developed a new version of the client, CMud. Unfortunately, by the time CMud came to exist, MUDs were on the decline, so the monies for further development were no longer there. It appears, however, you can still buy a license for ZMud today. For Windows only.
Atlantis - A client for MacOS, Atlantis is a full-featured graphical client that's been updated for Ventura (Apple's latest MacOS.)
TinyFugue - Another command-line client developed in the early 90's, TF (TinyFugue) died for a few years then was resurrected by a new developer and placed at GitHub with new features such Python support. You can read a brief history of TF at the Wikipedia. For Linux only.
Reddit MUD Client List - For more MUD clients, including apps for Android and iOS, the MUD subreddit has a wiki entry with an updated list of clients.
Grim Wheel is a site dedicated to old school text gaming.
Click the links below to go from section to section of this site.
Buildcraft - Articles which discuss the craft of building on Multi-User Dungeons (MUDs).
MUD Forums - Links to offsite forums that concern themselves with all things related to Multi-User Dungeons.
MUD-Dev - Where you can find archives of the former mailing list known as "MUD-Dev", as well as a link to the newest mailing list.
Imaginary Realities - Archives of articles from the now-defunct online magazine Imaginary Realities.
MUD Listings - Websites which list the many text games still in existence.
MUD Clients - Clients which you can use to connect to an active MUD.
Netbooks - A repository of supplements for tabletop roleplaying games.
Miscellaneous - Links that do not fall easily into any other category.
Codebases - The various public codebases in existence today.
Area Archives - Areas for public codebases such as Circle, ROM and Smaug.