I'm a huge fan of text-based MUDs because of the roleplay and gameplay mechanics they provide, and while usually I play Achaea, I've been hooked on Imperian (www.imperian.com) for a few months now. Imperian manages to have a great roleplay community and be PvP-friendly at the same time, it's a bit of a hardcore D&D type crowd.
I've been playing MUDs for 15 years and while Achaea is my all-time favorite, I settled into Imperian pretty fast. The newbie intro gets you right into PvE combat, helps you pick a guild+city, and the other players are generally helpful, especially your guildmates. I remember when I made my character, after hanging out with an older player for a bit they gave me a ton of gold to go and buy myself all the armor and equipment stuff I needed.
I know there's a ton of F2P MMOs with graphics out there to play, but MUDs have always held a special place in my heart because of the quality of players, consistent roleplayers, and gameplay mechanics that aren't feasible in graphical MMOs because of hardware limitations.
I invite everyone to check out Imperian and I'll try to help you out if you whisper me in-game, my character name is in my sig.