The original 40-button mouse


In the thread about Warmouse's unauthorized and many-buttoned OpenOffice mouse, Don Simpson points to ProHance's illustrious original. 40 buttons! It requires DOS 2.0. [AtariMagazines]

I have a 40-button mouse, the ProHance PowerMouse 100, from around 1990. ProHance Technologies in Sunnyvale, CA also made 3-, 12-, and 17-button mice.
If you think "ProHance" is so silly a name that no-one else would have used it, just try Googling it by itself