Conductive tape for biosignal acquisition
If you want to monitor electroencephalographic (EEG, electrical signals from the brain) or electromyographic (EMG, the electrical signals from moving muscles), you need to use electrodes to get the signal. The electrodes are generally not reusable and are kind of expensive. They usually run somewhere from $0.70 to $3.00 each, although the 3M Red Dot electrodes are much cheaper. The electrodes usually use a conductive gel surrounded by a ring of non-conductive adhesive, which makes them large and difficult to put close together.
It seems likely to me that you could use electrically conductive tape to improvise contacts of any size and shape. The adhesive probably isn’t tested for use on humans, and the conductive element is sometimes something that people have an allergy to, like nickel or silver, but it could be sufficient for self-experimentation for quick hacks. It would also be an order of magnitude cheaper
Conductive tape: http://www.tedpella.com/semmisc_html/semadhes.htm
Cheapo Red Dots: http://www.amazon.com/Red-Dot-Monitoring-Electrodes-Diaphoretic/dp/B000TBSAH0/ref=sr_1_1?s=industrial&ie=UTF8&qid=1336146625&sr=1-1