I used a TENS machine during labour. I couldn;t recommend it enough!!!

All four of my labours have been drug free natural births, but it was only the last one I used the TENS for. It gave me a real sense of control and reall did work to ease the pain, particularly in the early stages. There were a couple of times (near/during transition) where I questioned whether it was actually doing anything, so I stopped using it, and OMG there was an intense difference. I wish I had tried using one for previous births.

The TENs machine would work best with calm birthing methods or hypnobirthing IMHO.

My TENS machine cost me $25 on ebay (new from China). You don't 'need' a special pregnancy unit - you just need to ensure you learn how to use it and play with it before you are in labour and needing it.