Got around to building my stir plate. A standard LM317 build with an on/off switch.
There's an 80mm computer fan which as a piece of 1" schedule 40 PVC hotglued to it with some rare earth magnets on it. I've dremeled out some holes (melted out, really) for the magnets using a cutting wheel and then secured the first magnet with hot glue. The rest stay by themselves. The speed is controlled by an LM317 with a 330 ohm resistor going from OUT to ADJ and a 10k ohm audio taper potentiometer with on/off switch. I can control power on/off and get anywhere from around 3V to 11.5V out to the fan. The power connector is a standard kobiconn 2.1mm barrel jack from Mouser, but the rest of the parts are from Radio Shack. The box is a 6x4x2 project box from Radio Shack as well.
One problem I ran into was putting too many magnets on the fan. With 4 magnets the 1" stir bar would not spin at all. I think the friction of the bar pulled so hard against the device would just prevent the fan from spinning at all. 3 works great, doesn't throw the bar at even the highest speed, and is silent at low speed with a dimple being pulled into the starter.