Open Asset Import Library (short name: Assimp) is a portable Open Source library to import various well-known 3D model formats in a uniform manner. The most recent version also knows how to export 3d files and is therefore suitable as general-purpose 3D model converter. See the feature list.
AssimpView is a Windows-based model viewer. It loads all file formats that Assimp supports and is perfectly suited to quickly inspect 3d assets.
Assimp aims at providing a full asset conversion pipeline for use in game engines / realtime rendering systems of any kind - but is not limited to this audience. In the past, it has been used in a wide and diverse range of applications.
Written in C++, it is available under a liberal BSD license. There is a C API as well as bindings to various other languages, including Python and D. Assimp loads models into a straightforward data structure for further processing. This feature set is augmented by various post processing steps - ranging from general optimization tools to stuff like computing normal or tangent vectors.