There's a service called MaskMe that offers on-the-fly credit cards like this. It's a paid service, but what they do is you right click on a field, and generate a new card number. This card is basically prepaid - the service provider charges your real card for whatever the value you say, and the person you're buying from gets the number you just created.

Only real downside is that the generated cards are all MasterCard, and some merchants don't take anything but Visa.