Over the years I have built and run a multiplayer online game called Infinite Fear. It's a fairly medieval setting, in some interesting old fantasy world. There is the option of playing exclusively against the computer foes (of which there are thousands) or playing against other players as well. The gameplay is rich and complex, perhaps too complex, as it can take some people years to truly master it. But ultimately, if you enjoy fantasy and RPGs this game can be considered HIGHLY addictive.
The game-world is massive, with literally over 8000 rooms to explore, thousands of computer characters to interactive with, thousands of objects (such as equipment, scrolls, potions, wands...) to collect, and hundreds of spells and skills to learn, each unique to one of the 11 playable classes available.
One of the problems is you'll need a MUD client to connect, as it is a telnet program, employing ANSI colouring. Below I have put together a copy of GMUD for you, which includes a connection to my game already in it. If you wish to connect with any other client of your choice, the address is:
Host: puberdons.dyndns.org
Port: 4000
GMUD - This is a copy of GMUD with a connection to my game already present.