There is a guy who has about a dozen of apps in the store that (ab)use the headphone jack to power and control external hardware.

Among them is a laser pointer [1], a controller for RC power outlets [2] and countless others.

    [1] http://hmb-tec.de/iPhoneApps/LaserPointer.html
    [2] http://www.hmb-tec.de/HMB-TEC/Funksteckdose.html
    (Links are in German)