Buy King of Dragon Pass. It's not a MUD, but it's text-driven, and a great game on it's own right.

Whats a mud?

Adam Cardre's Photopia (scroll down on that page). Great fiction!

Multi-User Dungeon. It's basically a text based mmo.

I haven't played many of them, but I did enjoy my time with Achaea.

Well i never knew they existed. Kind of quirky, but i like the originality of it.

If you can get your hands on it, I recommend Planetfall.

As for MUDs, I used to play Valheru MUD a lot. That was great fun.

Zork? Available here on GOG
I used to play a mud pretty much full time for a year or 5 :P.
Ahh the good old days ;). But it no longer exists unfortunately.

A Mind Forever Voyaging is, in my mind, the definitive text adventure. Closely followed by 1893: A World's Fair Mystery, simply for the sheer staggering depth.

I also second Wishbone's recommendation of Planetfall, though you don't have to look far for it - it's included in the Zork collection right here on GOG.

Well, if you like adventure games and don't mind some realtime combat, Quest for Glory 1 & 2 are really great text adventures. You have to play the EGA version for text, though. The VGA versions are PnC.

This may have something of interest:

http://www.online-games.net/newadventuregames.html

The Interactive Fiction Archive has a massive library of text adventures. Depending on what format you choose, you may need to download certain programs (such as Windows Frotz) to run them.

Since the sheer volume of text adventures there is probably a bit overwhelming, I'll highlight a few:

Anchorhead: If you like Lovecraftian horror, this is a pretty solid entry in the genre.

Spider and Web: A spy-thriller told through flashbacks. Your character is strapped to an interrogation chair and is slowly reconstructing how they got there. I haven't played past the intro scene yet, but this one comes highly recommended by most people I've talked to about it.

Adventure: Colossal Cave: One of the early all-time classics in the genre. The puzzles are hardly intuitive, especially by modern standards, but it's a fun diversion, and a lot of early adventure games make references to it.

Slouching Towards Bedlam: Another game I haven't played, but have heard highly recommended. I think it's supposed to have a slight steampunk pastiche and some meaningful character choice options.

Finally, if you want to try your hand at writing some of your own text adventures, I highly recommend Inform 7. It uses a natural language coding system that takes a bit of getting used to, but once you're familiar with it, you can just write things like "The kitchen is a room. Bob is a person in the kitchen. Bob is wearing pants and the propeller beanie" and the engine will understand this and create all the appropriate rooms, objects, etc. The only thing I have found that Inform 7 is not very good with is tracking stats, should you want to use any, since you have to "describe" any math you want it to do in full sentences. I haven't used it in a long time, though, and it's possible that subsequent updates have improved this.

Man, it's been a while since I've played interactive fiction.

For a first Text Adventure I'd really, really recommend The Dreamhold, by Andrew Plotkin — it's specifically designed for newbies to the genre, without being condescending.

You can't go past Zork of course, but it can get devilish (and I loathe mazes with a passion, so adventures like that inevitably give me a headache). Still, nothing like Hitchhiker's.

In terms of the modern resurgence of IF, give some of Emilie Short's stuff a try, like City of Secrets. There's also Spider and Web or Slouching Towards Bedlam, which are great. In general, my favourite authors are Plotkin, Short and Cadre. Curses! is also brilliant, and more in the spirit of traditional IF, but again — some of those puzzles, man.

In my humble opinion, perhaps the greatest true text adventures were mostly Infocom...If you can find it, Wishbringer makes a pretty good introductory game to the genre.

Some great suggestions. I've downloaded a few of these ready to play when I have some time to kill and I have my netbook.

I've also starter HHGG again and it's amazing how fun such a frustrating game can be! I had forgotten how difficult it is!