Wikimedia
Wikipedia: Credit Card
This encyclopedia article describes how the credit card system works. It discusses the history, how credit cards are used, features, types, the numbering system, pros and cons, and more.
Texas Instruments
Texas Instruments: Numb3 Rs: "Uncertainty Principal" Checksum
This lesson plan introduced students to algorithms in the context of bank routing numbers (printed on checks). Students learn that the final digit in the routing number serves a specific purpose, and find such digits so that the routing...
PBS
Pbs: Secret History of the Credit Card
A site designed for teachers to accompany the PBS documentary by the same name. Lists of resources and handouts can stand alone.