A new online ultrasonics components store has just opened at www.UltrasonicsWorld.com. Check their amazing prices for replacement ultrasonics components, fully compatible with the major manufacturers' originals at a fraction of the cost.
(Now updated with a new link to NASA Telerobotics Program Plan, since the old page has been removed.)
An interesting page about research at MIT to develop a high torque-density solid-state actuator for use in the NASA/JPL Mars Micro Lander manipulator arm. It includes a general explanation of the principles of ultrasonic motors, and some cool animations! Note the use of dynamic and time-averaged ESPI (electronic speckle pattern interferometry) to evaluate the vibration modes.