Freyja is a 3d modeling system that provides content editing such as polymesh, UV, animation, and a wide range of model support via plugins. Many game character model formats are already supported.
Freyja also provides drop in libraries that allow you to use your content in your own game engine. The libraries use C++ classes and provide C bindings as well. A small STL, physics library, and material system expand the use outside just modeling content.