Adjustable "Sit-to-Stand" desks allow the user the luxury of both a sitting and a standing desk.  Adjustable Sit-to-Stand desks can be adjusted manually (usually controlled by a crank handle) or via electric motor.  At Standing Desk Nation, we are especially fond of the pneumonic manual desks that use counter-balancing to help you quickly and easily adjust the height of your desk, and maximize your ability to move throughout the work day.  Check out the Humanscale Float and SiS Ergo Constantforce for more info.