Adventure Quest:
www.battleon.com
Many people think Runescape is better, but it's an excellent time killer! It may not have the variation in things to do, but it's still fun.
Dragonfable:
www.dragonfable.com
Made by the same team who created Adventure Quest. The storyline happens before that on AQs and it's still not completely finished. There are lots of quests though and I still visit from time to time.