I saw Makerblock's topic about the ball joint, I thought openSCAD was suitable and advised it, but thought I'd give it a try myself. This is a very quick hack, but it can be easily modified since it's just code.

In OpenSCAD, you can choose to see an animation of the ball joint in action. Do this by uncommeting "demo();" and commenting "print();". To print it, choose the latter, press F6 and export to STL.

For a very cool implementation of this 'part' used in a camera mount, see thingiverse.com/thing:2631

Instructions

Modify it with the following parameters:

size=10; // size of the ball joint
jointspacing =0.5; // some space between them?
jointthickness = 2; // thickness of the arms
jointarms = 5; // how many arms do you want?
armwidth = 10; // actually: how much is removed from the arms Larger values will remove more

Derived from
https://www.thingiverse.com/thing:1967