My current favorite game:
Neverwinter Nights. I think the only game that I have as much play-time on is Descent. In addition to the excellent single-player modules that come with the game, there are many very good third-party modules available. This game never runs out of good adventures to experience.
The multiplayer is excellent. It allows one or more people to log into the game as a DM, which gives them a tremendous amount of control over what is going on. This allows for very open-ended and on-the-fly plots.
It follows a rule-set very close to Dungeons & Dragons 3.5, allowing me and my friends to play pen & paper sometimes, and then move our game into the computer and back again (as long as you are like me and are not too strict with the rules).
I will be making a list of my favorite modules before too long.
