Kinesthetic mouse feedback:

Give kinesthetic feedback by putting an electrically triggered brake
on the mouse ball that the system can trigger at opportune intervals. 
The system can give kinesthetic feedback of mouse movement
constraints, and can simulate the "feel" of a textured mouse pad.
An inexpensive piezoelectric mounted in an optical mouse might have a
similar effect. Seems like a natural place to put a microphone and a
speaker, anyway. Why not have two mice for a stereo effect?
As you drag the mouse over a region displaying editable text, you feel
the edge of each character cell as you move over it.
Modulating the volume for different "roughness" (or whatever effect it gives!).
Are there kinesthetically recognizable spatial textures?
Feel the edges of windows -- issue a tug upon enter and exit events.
Constrain mouse movement by putting on the brakes when it's moved in
an unacceptable direction. (like out of a region)
Could you "feel" an image by making a vibration whose volume is a function
of the pixel value under the cursor, whenever the mouse is dragged
over an image?
When the mouse was in a target, it would "sing" its (vowl, pitch)
location in the mouse, so you would feel and hear that you were over an
active button.