Suica and Passmo
A Suica or Passmo IC card can be used in convience stores to pay for food and drinks, to pay for train fares and many vending machines also accept them.
In a country that still doesn't fully accept the credit card (not many restaurants or convenience stores accept credit cards), IC cards have a growing popularity and are seriously convienient.
In Japan, like many other countries, you are expected to provide exact cash in most circumstances. Or at least it's much more welcome to pay exactly 563 yen in 500, 50, 10 and three 1s, instead of just giving a 1000 yen bill. So for someone like me who's lived in Japan for 3 years and still takes time trying to fumble around for exact change, my trusty suica card never leaves my wallet.
You can even top up the card in convience stores by asking to 'charge' the card with a desired amount while paying for your purchases. So, even if you forget to top up the card after taking the train to work in the morning, you can just do it while buying beer after work at the conbini.
ETC
Similarly, the ETC (Electronic Toll Collection) card has proved to be convienient for anyone using the motorways in Japan. Just having one of the cards installed in a ETC card reader on your car will allow you to continue through a designated ETC toll gate with no coins needed when your toll is paid. Making your travel that much more enjoyable, for those who don't want to spend a few seconds handing money to random people at least.
Some credit cards can be fitted with ETC usability too, so you can purchase various goods with the same card that pays for your tolls automatically.
