I use Square, and I like it a lot. You need an iPhone, iPad, or Android device. I use it with an iPad I got specifically for this purpose. People at shows love it... all the other vendors were using their phones, and customers commented how much easier it was to sign on the iPad. I doubt anyone would lose a sale over that, though!
With square you can also do non-swiped transactions such as over the phone, but the fees are a little higher that way.