I've been playing Neverwinter Nights recently. The single player campaign isn't as good in storyline terms as Baldur's Gate, but it is still decent. I think the implementation of 3rd. Edition D&D rules is very well done. Some people hate 3E, but I like it very much, so I'm am very pleased with this game.
Some people have prematurely called this game a Diablo/hack-n-slash clone... but I don't believe that to be true. There are lots of cool sidequests, tons of NPC coversations, class-based quests, etc. Plus the ability to have a DM is definately cool. I believe the game is very fun to play online with people who are serious about roleplaying their characters.
I beat the game, and have now gotten into some of the top rated mods out there. Some of them are really good. I can suggest "The Spires of Ravenloft" as a good one to downlaod. It got a bit tedious at the end, but it was fun.