Some times I need a joint.
This thing is one element of a One Degree of Freedom (1DOF?) joint. Put two of these together back to back, and put an axle through their joined holes, and you have a nice joint.
This thing, in conjunction with this other thing: http://www.thingiverse.com/thing:6354 make for an "arm" that attaches to a motor, and can be of any length (based on strength of motor and load).
1) Decide how big your rod hole and bearing hole need to be
2) Edit the OpenScad file to the appropriate sizes
3) Print a few of these
4) Press fit radial bearings, whatever you want, into the big round hole.
5) Press fit a rod or tube into the other hole
6) Connect to whatever you're connecting to
This makes for an easy way to form 'arm' segments. If one end of a bar is anchored to a servo, and the other end to this thing, then you have the makings of an upper arm, or a leg segment, or whatever.
You can use it any place you need a variable angle between two bars.