google is your friend,
www.buynfctags.com seemed to have low prices and a large selection just looking quickly.
I've thought/heard about similar uses but honestly I've always got my phone in my pocket, or have my hands full of other things to be swiping it when i get in/out of the car or my front door.
Combine NFC tags with payment systems and you've got a winner. (I want NFC enabled vending machines)
On a similar note I already use QR codes to simplify bitcoin payments, no need to type out an elongated payment address, just take a pic of the QR code.