NifSkope

OpenSource
http://niftools.sourceforge.net/wiki/NifTools
http://sourceforge.net/projects/niftools/files/

NifTools is an open source project creating tools—such as an editor, C++ and Python libraries, and import/export plugins for Maya, 3ds Max, and Blender—for files used by the NetImmerse/GameBryo™ game engine.

NifSkope
Useful for users/modders for games such as Morrowind, Oblivion, Skyrim, Fallout3, New Vegas, Etc.
Import/Export/Editing of Models