Oyster

London's brilliant transport card system, the Oyster card. It's a prepaid smartcard - you load it up with cash, and just swipe it on the way in and out of the subway or bus, and it figures out how much it costs and debits the money. The best part? If you do multiple trips in a day, you're guaranteed to never pay more than a daypass - the price is capped at the daypass price. I think there might even be something similar for weekly/monthly passes.

It's great for tourists - you don't need to learn all of the details of the system to use it. Sure, the subway is more expensive than the bus, multiple zones are pricy, and there's a premium for rush-hour travel. But I didn't need to know exactly how much each trip cost, or buy different types of tickets - just get on and ride.