Realm of the Mad God, that is. It has no real story, is just a bunch of mindless grinding, has killed my characters due to laggy connections, and is fun as hell to play. Really, I'm already worried about the time I'll be wasting on this one. But I just want to get into another God battle (I somehow made it into one, but didn't survive untill the end).
Here are some tips for new players: fight in groups for shared XP (less work to level up this way); your numeric pad acts as hotkeys for your items; teleport saves on travel time; and having to play this with a trackpad on a laptop is not ideal. :-)
Note: I didn't feel like trying this game until I remembered that DanC from Lost Garden worked on it. You may have seen me refer to his site in previous posts. There are some more articles on his blog which explore the design space around games like RotMG; good reading.