The Octopus card is one of the most clever tools I know as far as money is concerned. It was first introduced in Hong Kong in 1997 for fare payment on the MTR (the local transit system), but the use of the card quickly expanded for multiple other purposes. It can now be used to pay fares for majority of public transport in Hong Kong and to make purchases for consumer products at many stores in the territory. In addition it can be used in many soft drink vending machines, pay phones, photo booths, parking meters, and car parks.
Monetary value can be added to the card through a number of ways and you can add as much or as little as you wish. In MTR stations, enquiry machines can be found where cardholders may place their Octopus cards on the machines and the machines will display the balances along with a history of last 10 usages. You also know how much is left on the card (but without the last 10 usages) whenever you use the card.
It is widely popular in Hong Kong where it is estimated that 95 percent of the population aged 16 to 65 own at least one card and generates over 10 million daily transactions worth a total of about HK$29 billion (US$3.7 billion) a year. (edited and personalized from Wikipedia)