Transporter credits are a rationed unit of exchange that allows for use of transporters on Earth in the 24th century. They are used at Starfleet Academy. It is not known if they are used elsewhere on Earth.
In his early days at Starfleet Academy, future Captain Benjamin Sisko beamed back to his home in New Orleans so frequently that he quickly used up an entire month's credits. (DS9: "Explorers")
- The exact nature of transporter credits and the New World Economy remains largely unknown - though even in a society where money as we know it did not exist, it is not unreasonable to think that an institution like Starfleet Academy, which is in effect a military school, would only permit students finite use of something which can be used to leave the school grounds.