My Droid is one of the few so-called "2-3-2" capable robots. This refers to the ability to move from a 2 leg to a 3 leg stance and back, as seen in the original Star Wars movie (actually it is seen in only one scene, and only doing the 2 to 3 transition). This is done by extending the center leg from inside the body and pivoting the two outer legs, as shown below.
"2-3-2" is a difficult automation problem for an amateur robot, because of the large forces involved, relatively fast movement, restricted space, timing, and stability requirements. For this reason, there are few 2-3-2 droids around.
My system included a few innovations, including converting an existing club frame, using two independent, servo controlled linear actuators for the outer leg tilt, and a rather fast and very compact magnetic-locked center leg lift.
Because of the complexity and cost of the system, a parts run for the group was not attempted. However I described it in detail in my build log, and wrote a detailed document that was officially presented to the club at DroidCon 2011. This document is reproduced below. Many builders have since inspired themselves from this build, and reproduced, adapted or improved it.
CuriousMarc 2-3-2 Mechanism Overview.pdf