Check out this article on Turning an Old Hard Disk into a Rotary Input Device. Super geek stuff, but if you've got the know how and can do, it'd be a pretty tight piece of equipment (if only for the nerd novelty).
"A couple of years ago we built a fun system that would allow DJs to mix music tracks in an interesting way. Our design called for an input device that would allow the DJ to quickly seek through a track and find a specific playback position, and we wanted to be able to do this by spinning a rotary control with a flick of the wrist - much like turntable DJs can spin the record back and forth to do the same."
