FlxBox2D is a project mainly written in ActionScript, it's free.
Some classes to use Flixel with Box2D
This is my collection of class to use Box2D with Flixel.
B2FlxSprite is a FlxSprite with a b2Body atached to it. The shape is a polygon.
B2FlxTileblock is the same thing as B2FlxSprite but with a Tileblock and a static body.
B2Circle is a FlxSprite with a b2Circle shape atached to it instead of a b2Polygon shape.