The ultimate PONG
Griffin Technology (www.griffintechnology.com), a mac-oriented gadget company, manufacture an aluminium hi-fi-amplifier volume-knob on a USB piece of string.
It was designed for audio applications, though generally applicaple to any linear editing or navigating application
like jog-shuttling through video on Final Cut Pro.
There's a cool blue glow that emanates from the base so it looks just as good with the lights out.
Plugging in two powermates gives scope for two dimensional applications such as large plane exploration (e.g. gigantic map navigation). An extended one dimensional application, such as audio waveform navigation can use one for timebase scrolling, the other for timebase scaling (AKA zoom). Using a single powermate for zoom is surprisingly useful in applications such as Adobe Photoshop.
Technically speaking, the powermate registers itself as a one-axis mouse with a single button (you can click the knob down).