Box2D is a free, open-source 2D rigid body physics simulator for games. It is widely used by indie and mobile game developers due to its efficiency and accuracy in simulating realistic physical interactions. Box2D is known for its robust collision detection, joint constraints, and dynamic simulation capabilities, making it a powerful tool for creating engaging gameplay mechanics. Notable games built with Box2D include the immensely popular Angry Birds franchise, Snail Bob, and many others that leverage its physics engine to deliver fun and challenging experiences.