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.

Bank of America offers this as well. They give you the option of specifying a one time limit, monthly limit, and expiration dates for the number.

Citicard has a virtual credit card number which we can use for a certain period of time. Never used the virtual number myself though.

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.

Paypal does this very easily online.

Speaking of evil companies....