If you're looking for something like an MMO, try "The Realm." It used to be made by Sierra but was sold to Codemasters, and I have no idea who owns it now... could still be Codemasters. My dad and I used to play it back in the day when we had two 56k connections, but we finally got bored and moved on one day. That's a $5 a month for playing, though. As far as I know the game is still up and running- I almost started playing again late last year.
Another good one is "Dark Ages". Not the Dark Ages of Camelot, but "Dark Ages," by Nexxon. I think that's $10, but fully playable on dial-up and is really fun if you get into the community aspect of it (you can level up by teaching classes on poetry... I did that once
)
If you're meaning multiplayer of any type, then just look around- there's plenty of em.
Cheers,
-Fae