GSR App for Iphone, requires a small hardware addition to run.
Wouldn't it be interesting if you could create the classical galvanic skin response, or e-meter used by scientologists without a circuit? This is one way to do that using low frequency bioimpedance spectoscopy(50Hz) AC sine waves instead of the classic DC. Check it out!
Here is a link to the original instructables article with details on how to make the electrodes. http://www.instructables.com/id/IPhone-Excitement-Meter/
More instructions on how to make electrodes: http://www.instructables.com/id/Making-Galvanic-Skin-Response-Finger-Electrodes/