For my birthday, my father in law Mike, got me one of those robot arms from Maplin. It just so happens that this is exactly the same arm that others have been hacking using the Raspberry Pi for a long while now, so guess what? The hard work is already done!
Building the arm took a few hours, and all you really need is a small cross-head screw driver and 4 D cell batteries.
Looking at the base where the batteries go, it is the perfect size for a Raspberry Pi, which would replace the batteries. Also, to make it a more complete and portable package, I decided to investigate the possibility of a single power supply for both the Pi and the arm. The D cells are 1.5v, so coupled together make 6v in total. Ok, so the Pi has 5v direct output, but I really don't want to pull the kind of amps required for motors through the Pi, so we'll leave that well alone! Instead, I powered up my bench PSU, set it to 6v, 2 amps max and applied power.