DoktorAndy built this scratch controller made from parts of an old harddisk and an optical mouse:
Check out this cool Hard Disk Drive Midi Scratch Table. It uses the sensor from an optical mouse to look at the movement of a hard drive platter. There are a number of buttons that can change the midi output of the device to allow it to adapt to your DJ needs. The optical sensor is used to read a pattern on the underside of the platter to determine when the user is spinning it.
Via Hacked Gadgets.