I've often thought that computer mice would be useful pre-made input devices for movement, and button sensors, so this is just a start to collecting information about them.
Potential uses for computer mice:
Rotary optical encoders using mechanical mice roller wheel sensors
eg robot motion (direction and distance) feedback
eg wind speed indicator
eg dynamometer
eg speed measurement
Use the whole mouse (either mechanical or optical) as a position tracking sensor that supplies motion X-Y info