Fingertip ring USB Optical Mouse

This mouse is supposed to solve the problem of space at the office or
when you travel by train, bus or plane. Works by strapping to the
index finger with a button operated by thumb and a scroll wheel as
well.

They point out that it can get a while to get used to the idea of
vertical finger operated mouse, and that it's not for applications
that demand precision like photoshop or action games.