Box2D was originally released as Box2D Lite in 2007. It's extremely popular among indie's and has been ported to a bazillion languages off of C++, most popularly through Flash AS3 and Java. It runs on nearly every Major Operating System, from Windows to Blackberry 10.
- All Box2D objects are b2body's, which have intrinsic velocity, density, friction, and restitution. This allows several objects to interact with others.
- Simple Shape and Joint system for attaching bodies together, including ropes.
- Any inactive object is put to "sleep" until impacted by another active object. This allows the engine to conserve memory and have several more objects on screen then are actually being managed at any one time.
- Liquid bodies can have Buoyancy which automatically generates waves as well.
- Rotational gravity allows objects to generate their own gravitational forces, Super Mario Galaxy style.
- Streamlined Raycasting, which allows for better line of sight detection.
Since Box2D is released as Open Source, it is used in several commercial games.
Rovio famously used Box2D to develop Angry Birds, which has inspired several "physics games" such as Happy Wheels which use Box2D as their centerpiece. Other games use it slightly more discreetly, such as Intrusion 2 which uses it to create a physical world in an otherwise Metal Slug-esque gameplay style.
The fairness of using a physics engine for a physics game has come into question, but Erin Catto famously has given Rovio his approval. Despite all this, he has stated that he desires a Blue Sweatshirt since the Red Sweatshirt Rovio sent him as a gift is actually his least favorite color.