Using not much more than an Arduino, a piece of metal shim, and a bunch of wire, Instructables user sketchsk3tch built this Arduino Magstripe Emulator, which purportedly can spoof a magnetic card reader into thinking a card has been swiped. Quite the MacGyver solution to magstripe hacking…

More:

• Magnetic stripe card spoofer
• Decoding magstrip cards with Arduino
• Powdered rust reveals magstripe data
• Make your own Square credit card reader?
• “Banned” slides — The Anatomy of a Subway Hack: Breaking Crypto RFID’s and Magstripes of Ticketing Systems