Like a bunch of people have already said,
MapleStory. I played that for aaaages, but I don't really know why. It's pretty much just killkillkillkillkillkillkillkill and if you're lucky you might level.
More recently I've been playing
Mabinogi, which still confuses the hell out of me, but it's pretty fun. Lots of dungeons.
FlyFF is another, I had a lot of fun playing that with my friends.
It's probably easier for you to look at the sites and decide which one/s you want to play.