A stir plate is a device for constantly agitating and aerating liquid; it is most often used by home brewers for adding oxygen to a yeast starter.
A stir plate consists of a base unit and a nonreactive metal rod, sometimes covered with rubber or plastic to prevent it from scratching the vessel containing the liquid. The vessel, often an Erlenmeyer flask, is placed on the base unit, and the rod is placed in the vessel. A magnet in the base unit then moves the rod around in a circle, stirring the liquid in the vessel.
A simple stir plate can easily be built from a CPU or power supply fan, a 6-12 volt AC-adapter, some neodymium magnets from the local hardware store; and a box to house the fan and place the vessel on. A sawed off piece of a stainless screw or bolt can be used as the stir bar, preferably encased in heat-shrink tubing so it doesn't rattle around. A bar made from small "button" neodymium magnets also works well.