box2d is C++ library - known as box2d v2.x
Collection of graphical frontends for box2d debug rendering.
To ensure best precision box2d uses units called meters which is just a fancy name for small floats because floating-point numbers lose precision when the number gets bigger. That's why it's the best to keep size of your physics object in range from 0.0f to 10.0f and then use scale while rendering.
MIT, see license for more information.