Griffin's PowerMate USB controller may be a few years old, but it still manages to pull in buyers. Anyone not willing to spend $45 on a glorified volume knob, though, might find something to occupy them in MAKE's weekend project, the Powerfake.
Constructed from the scroll-wheel from a PS/2 or USB mouse, a remote control car wheel and tire, and a small project box, the Powerfake basically replaces Griffin's mechanism with a standard mouse scroll mechanism. It's up to you how glamorous you then make it, either leaving it relatively bare like the one in the picture above, or dressing it up with a chromed dial.
Full instructions are in the MAKE PDF, available to download here [PDF link], and depending on what you have lying around the house or garage, it should be a relatively cheap hack too. Anyone planning on attempting this over the weekend?