QuickB2 is a complete abstraction of Box2D, providing grouped hierarchies, extensible classes, and event-driven callbacks. The high-level interface enables developers to create physics simulations that are impractical to write using Box2D's API alone. It takes care of things that you don't want to, so that you can concentrate on your game.
I've just gone and looked through the demo reel, and I have to say this is quite impressive. Obviously having a nicer to use, ActionScript 3 style syntax rather than the C++ syntax is a bonus, but what really makes this cool is the features that this library has, over and above the basics of Box2D. Things like shape cutting, soft bodies; breakable joints etcetera.
You can find out more information, and download the source code from Google Code, at: //code.google.com/p/quickb2/