A challenge

You've now built a two-legged robot, and you can easily expand this to robots with even more legs. Here is an image of the mechanical structure of a four-legged robot that has eight DOF, which is fairly easy to create using many of the parts you have used to create your two-legged robot. This is my personal favorite because it doesn't fall over and break the electronics:

You'll need eight servos, and lots of battery. If you look on eBay, you can often find kits for sale for four-legged robots with twelve DOF, but again realize that the battery will need to be much bigger. For these kinds of applications, we often use remote control (RC) batteries. These are nice, as they are rechargeable, but make sure you either purchase one that is 5 to 6 volts, or include a way to regulate the voltage. Here is a picture of this kind of battery, available at most hobby stores:

If you use this type of battery, don't forget a charger. The hobby store can help with choosing an appropriate match...