GLUI is a GLUT-based C++ user interface library which provides controls such as buttons, checkboxes, radio buttons, and spinners to OpenGL applications. It is window-system independent, using GLUT or FreeGLUT.
NEW: Bullet is now hosted at http://bullet.googlecode.com
A hight performance well designed Collision Detection Library for the Open Dynamics Engine. Supports CONCAVE TRIMESHES and Deformable bodies.
IBDS is a library for dynamic simulation of multi-body systems in C++. It simulates rigid bodies, particles, many different joint types and collisions with friction. (ZLib license, free for commercial use) More infos: http://www.impulse-based.de
QUDAL - the Quantities, Units and Dimensional Analysis Library - allows you to work with physical units (SI and otherwise) in C++, as well as with quantities measured in such units.