OGRE (Object-Oriented Graphics Rendering Engine) is a scene-oriented,
flexible 3D engine written in C++ designed to make it easier and more
intuitive for developers to produce applications utilising
hardware-accelerated 3D graphics.
Open Dynamics Engine (ODE)
ODE is an open source, high performance library for simulating rigid body dynamics.
It is fully featured, stable, mature and platform independent with an
easy to use C/C++ API.
Crazy Eddie's GUI System is a free library providing windowing and
widgets for graphics APIs / engines where such functionality is not
natively available, or severely lacking.