I'm gonna list all of the Massively and not so Massively Multiplayer Online games I know of.
Maple Story is a Side-Scrolling MMORPG. Pretty fun if you ask me.
GunBound is like the game Worms, but online and with alot of different vehicles. I used to play it ahwile ago and recently tried to get it again. But it doesn't work on my computer.
Survival Project can best be described as The Legend of Zelda: Four Swords Adventure online.
GunZ The Duel: International Edition is a game I have yet to try. From what I've heard, it's an MMOFPS. After I try it, I'll edit this with what it really is.
That's all I can think of right now. Feel free to add to this.