If you've never tried a MUD, there are still a few out there that are alive and kickin'. Funny enough, I've been scratching that itch over the last few days and seeing whats out there. They're something like a pre-cursor to MMOs - online, text-based games. If you get really deep into stuff like PVP, you'll like wind up writing scripts that trigger actions based on what's happening since its quicker than typing out commands when things get hot and heavy.

If I had to guess, I'd say Aardwolf is probably the most populated and has the most users online at any given time. I have an old char on there that I occasionally log into and run some quests on:

Aardwolf

I just created a character in Alter Aeon and it's alright so far, but I haven't spent more than about an hour logged in:

Alter Aeon

I don't know how people generally feel about Iron Realms Entertainment. Some or all of their MUDs end up with you kind of having to spend some money if you get super engaged, but I'm pretty sure most of their games are perfectly fine without paying for casual players. They have a handful of MUDs that cover different themes (classic fantasy, vampire stuff, etc). I actually tried out Starmourn recently which is a sci-fi themed one, but I think they're no longer developing it actively - the servers remain up (for now, at least, I guess). Regardless, all of their games seem pretty polished and thoughtfully made.

Iron Realms Entertainment main site

Starmourn

The cool thing about IRE is that their games are all playable in a browser and the browser-based apps include some QoL UI stuff like maps and stuff. The others generally require a (free) MUD client like Mudlet. Aardwolf has a highly customized version of Mudlet that has frames/windows within the client that show you your characters stats, maps, a chat window, and some other stuff.