I've been using Apple's Mighty Mouse on my desktop machines for a couple of years now. I quite like them, although the mouse's inability to represent both mouse buttons being held down at the same time makes it necessary to keep a conventional mouse around for things like gaming.
This is a nice mouse to use, though. For example, it makes a nice solid mechanical click when you use the left or right buttons (even though there is really only one mechanical button — the whole mouse — touch sensors inside give you two "logical" buttons).
There's even a tiny clicking sound when you squeeze the side buttons or roll the little trackball around. You can hardly hear these sounds in a normal office, but they make all the difference to the "feel" of the device. And, until today, I would have meant that literally: I'd have sworn that I could feel the little clicks through my fingertips.
Today, quite by accident, I discovered that the mouse does not make these tinier sounds if it isn't plugged in… or, in the case of the wireless version, if you take the battery out.
Yes, there's a tiny speaker inside, whose only purpose is to make sounds that are almost — but not quite — too quiet to hear.