The Nebula Device
The Nebula Device is a free multi-platform game engine that currently runs under Linux and Windows.
It uses OpenGL or Direct3D to render and TCL/Tk for scripting.
Living C++ objects can be browsed and manipulated at runtime from the built-in console, or remotely from a minimal terminal-based console application. A powerful channel animation system allows any floating point attribute to be animated, and specialized animator classes allow things like real-time bone animation with weighted vertex skin.