OGRE 3D
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.

CEGUI
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.