Trackball
In computing, a trackball is a pointing input device. It consists of a ball held by a socket containing sensors to detect a rotation of the ball about two axes.  It is like an upside-down mouse with a ball that sticks out.  The user rolls the ball with the thumb, fingers or the palm of the hand to move a cursor and can click one of two buttons (identical to mouse buttons) near the trackball to select desktop objects or position the cursor for text entry.  Currently only three major companies: Logitech, A4Tech and Kensington make many of them.  Microsoft was once a major producer.