Citibank provides a web service that dynamically generates a credit card number linked to your account, which is only valid for a month. This service comes free with their credit cards AFAIK.
I like the "Paypal plugin". You can generate both temporary long-term and temporary one-time use cards. Very handy, and you have the extra protection of having it linked to your Paypal account, so if you choose to limit your Paypal balance to $100 you don't have to pull any tricks like you would with a bank account.