What in the name of all that is good and holy is a MUD? Are they still actively played?

So, I have recently become addicted, so to speak, to this genre of video games called "roguelikes." I don't know what it is that is so appealing about them. Maybe it's the randomness of the dungeons. Maybe, for some, it is the huge and random, endlessly interesting worlds that are generated with each character. Maybe it's their simple design with endless levels of intricate complexities that make them a wonderful chore to dig into and cherish every moment of.

Then I began, in my perusing of the internetz, to see these things called MUDs.

Multi-User Dungeons.

A roguelike, but -- like, RuneScape/EverQuest/WoW/whatever multiplayer?! epicface.jpg

Are there any really popular roguelikes out there that are definitely worth a go? Deep worlds with thousands of players, quests, epic story lines, grouping, politics, etc?

Thanks in advance. :D