Kensington Orbit Optical Trackball - Trackball - optical - 2 button(s) - cable $27.99

Control your cursor with a simple touch of the finger! The highly accurate Orbit Optical is as easy to use as a regular mouse, but it stays in one place saving you valuable desk space. The precise optical tracking mechanism is also resistant to dust, reducing the need for regular trackball cleaning

Buy.com is carrying this Optical Trackball for $27.99