Square. I use it on my phone with a credit card swipe (they send it free). There is no monthly rate, only a pay per swipe charge, and I have the cash in my bank account in 24 hours.
https://squareup.com/
I never had a bad experience with them. I swipe the card, the customer sees how much they are paying they sign my droid phone with their finger, and I process the payment. Pretty simple and 100% portable. You can put the card number in manually also if you want, but they chare about 1% more for this method for some reason. It accepts all major credit cards, and no extra for Amex (unlike most other credit card processing).