Hi all,
Recently I uploaded a small lib for 2d transformations (here). Now I made something similar but for 3D 
Basically it implements a Node class with transformations that you can attach to other nodes and build your scene graph (like building a skeleton out of bones). Then you add Entities to Nodes to actually draw stuff. The Node class handle the transformations, updates, caching transformations, etc.
The lib provides the absolute basics for a scene graph and have a very simple API. You can get it here:
Cheers!