Cartoon Network has launched its browser MMO, Fusion Fall. You can sample the game for free at the Fusion Fall website.
With a time-travelling storyline involving Dexter, Samurai Jack and other toons from the popular kids' channel, Fusion Fall seems aimed squarely at tweens. You can play the early levels for free - after that it's USD 5.95 a month, or USD 9.95 for a "family plan" including four accounts.
Fusion Fall is a full 3D game with a colourful, cel-shaded, platform-game look, but it launches pretty swiftly in a browser window. If you're older than 12 it will mostly be of interest for checking out the impressive Unity web platform. Or meeting Samurai Jack, who is awesome.