Nevergrind is a very addicting ARPG/MMO/Competitive RPG mash up!
There are competitive ladder seasons with softcore and hardcore leagues. Developed by a guy who was in the U.S. Navy with heavy inspiration from Everquest and Diablo type games.
Its simple yet awesome! Random loot just like an ARPG, the simpleness of a browser game, and the complexity of well defined RPG in terms of character building. I suggest at least trying it!
Nevergrind - Browser RPG
(Official Game Trailer!)
Its also on Kongregate!