Knob controllers can control volume settings, scroll through web pages and help edit movies among other uses. You can buy a commercial one from Griffin Technology for $US45 or build one yourself with an old scroll-wheel mouse and a few electronics components and tools.
DIY enthusiast zine Make offers instructions and a video tutorial on how to construct the rotary knob controller using a PS/2 or USB mouse with scroll wheel. The scroll wheel will need to work via a rotary encoder and not an IR transmitter/detector; see the source post or the video tutorial for mote details and how to spot the difference.
Once you have a suitable donor mouse with a rotary encoder, you will gut the mouse removing the scroll wheel, rotary encoder, cord and circuit board. After that you'll desolder the encoder from the circuit board, add a small metal rod to control the encoder, mount the parts in a project box and attach a circular object that rotates to control the device -- the creator uses a remote control car wheel.
With the controller assembled you can now shut down your computer, plug in the PS/2 plug from the mouse and boot your system. Your computer will recognise the controller as a mouse automatically so after adjusting the sensitivity of the wheel go ahead and try it with web browsers, music players and any other applications you'd normally use a scroll wheel.
To get more functionality Windows users can use the free program Volumouse to assign volume controls, adjust screen transparency and brightness, and other functions.
PowerFake Knob Controller [Make Zine]