Transcutaneous electrical nerve stimulation or as they are more commonly known TENS Machines can be found online, at medical supply companies, drugstores and at many conventions and state fairs. As an example, the banner pictured was at the recent California Restaurant Association convention and show at the Los Angeles Convention Center.
TENS uses a small electrical current (usually supplied by batteries) to conduct electrical current through your muscles in an effort to reduce and/or control pain. By placing the electrodes along an area where you are experiencing discomfort, the TENS sends electrical pulses along then nerve fibers.
Some people have experienced pain relief; although the theories on why are a little fuzzy. It may be that the electrical impulse scrambles the pain nerve signals before they reach your brain. Another theory is that TENS main induce your body to produce natural pain killers or endorphins.
Of course, if you are experiencing long-term, chronic or severe pain, you should consult a doctor.