A guy I do work with used it for a Flash game he developed and we played around with it on a webpage for his games.
I knew Angry Birds used it because the head of Angry Birds spoke at the GDC and the guy from Box2D asked why Box2D wasn't in the credits for the game and Angry Birds guy admitted that it should and would be.
Erin: “Hi Peter, could you tell me which physics engine Angy Birds uses?” Peter: “Box2D” Erin: “Great. Would you consider giving credit to Box2D in your game?” Peter: “Yes, of course” Erin: “Thank you! By the way, I am Erin Catto the creator of Box2D” Peter: “Great! I would like to talk to you after the session”