I found this very useful list of 25 free 3d modeling programs.
Actually I was googling for the TopMod3D which I had
previously installed on my old pc. Now I have a laptop
which has got no TopMod3D installed yet.
It´s very easy to make complex
high genus 2-manifold geometry with it and with this tool you
can create artistic solid (watertight) mesh forms for 3d printing
and prototype models.

I have had so much fun with it and
created a basic yet a complex shape which I then exported
into Cinema 4D and added hypernurbs and with some further
subdivision modeling on the inner parts. And of course added the
sphere inside.

Here´s some tests made with TopMod3D and Cinema 4D.