In the past few months, we have covered iCub a lot here. It is a humanoid robot developed by the Italian Institute of Technology. The latest video by these guys shows the robot balancing on two feet and even one foot. iCub stays balanced by regulating the interaction forces between itself and the environment:
As you can see, the robot can maintain its balance on one foot or two feet easily. The researchers claim the robot can keep its balance even when strongly perturbed. You can find out more about it here.