In addition to purely practical things like cases for electronics and hangers for air tools I've been experimenting with recursive generative art. The 'alien globe' takes on many different forms depending on the parameters, sometimes tree-like, sometimes like blood vessels and sometimes, well, just alien.
One of the challenges with generating random trees for 3D printing was coming up with an algorithm that keeps the angles from being too horizontal. The shape has to support itself as it prints. It would be nice if we had four or five axis three-D printers that could overcome this limitation.
Another challenge for generative 3D models is creating smooth 'knuckle' joints - you need a sphere at the intersection of two cylinders to close up any gaps.