Platforms: iOS, Android (both free)
What's better than recycling receipts? Not producing them in the first place. The Square Register mobile app works with a miniature credit card reader that plugs into your tablet’s headphone jack and lets you accept credit-card payments on the go. The app and card reader are free, but Square does charge 2.75 percent per swipe for Visa, American Express, MasterCard, and Discover cards. To accept a payment, just swipe your customer’s card through the Square card reader, and ask them to sign on your tablet’s screen. If desired, they'll get a receipt emailed to them.