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 a general-purpose 3D model converter.
Known 3D engines have their own Asset loaders
If some unifies would be better
Work has just begun. Engine-agnostic Loader in Haxe
Do not hesitate to offer any help: pushes (any thing, it doesn’t matter), coding,testing, website, wiki, etc