CircleMUD is a DikuMUD-derivative MUD environment, which basically means that it's a *NIX program that runs constantly and lets people telnet into it so they can hack and slash their way into a lot of wasted time.
CircleMUD was developed by Jeremy Elson, a student at Johns Hopkins University, and was first released in January 1992. Elson took the DikuMUD codebase, which was open source (even back then!), and basically just modified the hell out of it, so much so that it couldn't rightfully be called Diku anymore. So, Elson named it CircleMUD and gave it to the public. It was also an open source release. The very first version of Elson's MUD under general use (prior to the release of the code), imaginatively titled CircleMUD, ran on Ultrix 4.0, on a DECstation 3100 at the JHU computer science lab, and supported a maximum of 58 simultaneous connections. This number grew, naturally, as the CircleMUD code became more complex and bandwidth and disk space became more readily available commodities.
CircleMUD was not originally open source. It remained just a project of Elson's until mid-1993, when, while skimming through posts on rec.games.mud.diku, he realized that there was no One True Code for MUDs -- they were all completely different, mostly non-portable, and most took months of tweaking to get what you wanted out of the code as an administrator. And so, he fluffed up the CircleMUD code a bit and released it to the public domain. It was an instant success. The first public versions ran on UNIX derivatives only, and the code was eventually ported to Linux, MS-DOS, *BSD and a host of other operating systems. Prior to CircleMUD, even little bits of code wouldn't work on one system when they would on another. For example someone running Diku on a SunSPARC could write up a fancy little program to make NPC thieves randomly pickpocket you. However, if this SunSPARC guy gave that code to someone running Diku on an SGI running IRIX, the code wouldn't work. CircleMUD solved most of these problems by integrating a standard codebase that was shared and contributed to via the JHU FTP server, and, eventually, the circlemud.org FTP server.
The first release of CircleMUD, while successful, still left a lot to be desired, and by the time 2.0 was released much had been changed, largely thanks to independent MUD administrators who wanted to see their unique little features become mainstream. The release of CircleMUD 3.0 introduced OasisOLC, (OLC being an acronym for "OnLine Creation"), which allowed for realtime editing and creating of whole zones, rooms, NPCs and objects, without the need to compile them into the codebase or reboot the server the MUD was running on, both of which were necessary in pre-3.0 versions.
As it stands now, CircleMUD is probably the most widely recognized and most commonly used MUD code out there, at least among the Diku derivatives.
MMORPGs have pretty much taken the place of MUDs as of 2001, and most players of MMORPGs aren't aware that the telnet protocol even exists, particularly now that ssh has made it obsolete. Like the BBS, MUDs are mostly an artifact of the past, though if you look hard enough you can still find some rather good ones.
The only MUD, Circle or otherwise, I ever got into was The Age of Insanity, which I visited regularly as a player for two years, and as an immortal and later a god for four years. It's still around, surprisingly enough, although nobody really plays it anymore. It's at ageofinsanity.org, port 8081, if you want to check it out.