I would immagine that almost every non-trivial robot you find in a research lab today includes a Kalman filter somewhere.

I know the balancer I worked on (https://collab.cc.gatech.edu/humanoids/node/1241) certainly does, although from that project I can also tell you that most Kalman filter implementations are probably incorrect. It's fairly easy to implement a filter which behaves quite well but does not actually behave like a Kalman filter.

That's one piece that's often used in control systems, but the clever thing here is the control of the 2-link pendulum, by reaching a desired energy first to limit the search space.