I wanted to be able to rotate two different LEGO assemblies around the same axis so I created this customizable hub. It is based on my Customizable Curved Beams for LEGO Technic http://www.thingiverse.com/thing:629875. Even though you could create a complete circle from the curved beam design, this is a more efficient approach.

This thing is customizable either with the center solid with a cross to hold a LEGO axle or without as just a ring. The number of holes can also be selected which determines the diameter and the degrees between holes.

I used these hubs with a LEGO Mindstorms EV3 set to create a kinematic art work. You can see a photo above or watch it in action at https://vimeo.com/120904973

Note that depending on your printer and how tight you want to make the fit for the axle, you may need to adjust the size of the cross slot. This can be done by editing the OpenSCAD file directly.

Update 3/2/15: I discovered that an even number of holes might be desirable for some mechanisms so I added several in the number of holes option. I had to tweak one line of code so the diameter of the inside set of holes will always be a multiple of the Technic beam hole spacing (8mm). If you want another hole count, just edit the OpenSCAD file directly.