A smart card that can pay for fares on various types of transportation in Hong Kong: MTR, KCR East Rail, KCR Light Rail, KMB, NWFB and First Ferry routes to the outlying islands. These cards cost HK$150 and this includes a HK$50 deposit, refundable when you're realize you're never going to travel again in Hong Kong for the rest of your life.
The cards are extremely convenient. They're the size of a credit card but several times thicker. When you enter the bus, MTR station, etc., you swipe the card (or wallet, purse, or whatever you keep the card in) over a special plate and the fare is either deducted immediately or your point of boarding is registered, in which case the fare will be deducted when you leave. You can recharge the card at Add Value machines, which are located in MTR stations and 7-Elevens.
Personalized cards are now available. For a HK$20 handling fee, you can have your photo placed on the card. Since no one ever looks at the cards, I'm not sure why anyone would do this.