It's not like I tried too terribly many, but I used to be awfully fond of the El Dorado MUD (telnet://eldoradomud.com:9000). I joined the Knight Templar guild years ago, and was the librarian or archivist or something like that -- I kept a notebook of all item stats that I ran across. Anything I found, I identified, and kept a record of what it could do, what it was worth, etc. That way, if somebody found something new on a quest, I could give them an idea of what they had.
Cool. It's always nice to see another MUD veteran floating around. My favorite MUDs are/were Stick in the MUD and CthulhuMUD. But I'm mostly a MUSH guy myself. If you'd like to connect to my MUSH, it's cavemush.com 6116.