Dan´s Data has posted a review on the IntelliMouse Optical.
Microsoft´s IntelliEye line of optical mouses are quite popular, and for good reason. They´re the first no-ball mouses that don´t need a special mousemat.Read more
Most mouses are "opto-mechanical". They use a rubber-coated ball that turns two little rollers, which are on axles that end with a slotted disc, which interrupts a couple of little beams of light, which in turn allows the mouse´s electronics to figure out which way the ball´s turning, and how fast.