The spherical 5-bar linkage

I've been working on one, and amused to find that the physical construction and control software took about the same amount of time. Here's a movie, under control of the mouse in the other hand:

The idea of a spherical linkage is that all the motion takes place on the surface of an imaginary sphere, but in reality the links can be at different radii as long as their motion is well-described by rigid shapes moving on the sphere.  That makes it possible to construct in the real world.
There's some fairly cute math involved in converting a position you want the end axis to be, to angles to set the driving servos.  Quaternions are involved.