Trans Electronic Nerve Stimulator or something like that... My Godsend!
It has a 9v battery in a little rectangular thinger that clips to your belt (or that you hold in your hand when you have no belt!) and attaches to 4 electrodes (like those sticky patches they use in the hospital) that you stick where you hurt (in my case the lower back) it delivers a little electrical shock (it feels like a mild buzz to a pretty good buzz depending on how high you turn it up) and works on the pain gate theory... your nerves can only carry so many signals so if you overload them with a mild electrical current you feel that instead of a pain signal from a contraction. It worked so well for me. Before sticking it on I was in agony and Jesse was pushing on my lower back with all her weight. When I had the TENS on I was able to deal with it on my own. So good! I would never do labour without access to one. Our midwife had one for me to use but also knew where to rent them if hers was out of the office with another client.
They have no impact on the baby and no lasting effect on Mom. They're also used in Physio to stimulate nerve endings I think.
At any rate, I would NOT have made it through labour without it! You can find more info at wikipedia