Virtual credit card number

My Citibank MasterCard allows me to generate a virtual credit card number, pretty simply, using a fairly ugly web interface. But a few clicks and you get a picture of a credit card, including numbers, dates and CVC. The magic is that this credit card can be used only once so if it's stolen you don't run a risk. Pretty cool. I just used it to pay for some software online and it worked like a charm. Simple and useful!