The Z-Nano optical mouse is, according to its makers, the world’s smallest mouse–and it’s literally no bigger than your thumb. That is if your thumb is larger than 42 x 21 x 17.65mm.
Tiny though it may be, it looks like it’s actually a well-designed midget gadget. It’s got 1600DPI laser tracking resolution (user selectable, no less, from 100DPI upwards). There’s an internal LED light, scrolling controls built-in, two button-clicking, a magnetic cable-management setup and a tiny slot so you can customize it with a teeny photo of whatever tickles your fancy.
It’s undoubtedly an innovation on the road to the miniaturization of everything, but it’s a significant $60 though, so size better be important to you. Different people’s choice of pointing device tends to be highly personal, and with my awkward thumbs and worries about muscle cramps I’d never consider using this. Your mileage may vary, of course, as will your thumbs.