The Freyja modeler is written in Gtk+ 2.x with the Mlisp domain language binding, so you can alter the interface to your needs without recompiling the modeler. Current platform support is for Linux amd64/i386 and Windows. Freyja also provides drop in libraries that allow you to use content translation in your own game engine. The libraries use C++ classes and provide C bindings as well. A small STL, python wrapper, physics library, and material system expand the use outside just modeling content. This project is still in early development in terms of use for the end user due to it's past focus on reverse engineering 'cooked' model formats.

If you'd like to get involved or just learn more about it -- visit the forums on the homepage.