The CMU Robotics Institute has released a new technical paper title, "An Actuator with Mechanically Adjustable Series Compliance" (PDF format). It sounds complicated but basically what they're describing is the use of a spring with adjustable stiffness in the leg of a biped. The spring is adjusted to recover and reuse energy that would otherwise be lost when the robot is running, jumping, or performing other high-energy tasks. The paper includes plenty of background info, diagrams, and a photo of the complete leg actuator. The research is being done at the CMU Microdynamic Systems Lab.