You can tie to any credit card or checking account. It creates not only a virtual credit card number but also a faux e-mail address, name, and other contact information for use with the number. You can create such profiles either at the company's site or through a Firefox add-on (installed as part of your signing up at the site; an add-on for IE is in the works). Once installed, the add-on recognizes shopping sites and autofill forms. ShopShield offers a free level of service linked to checking accounts; a pay-as-you-go service for debit and credit cards ($1.99 per use, plus a $0.49 charge for transactions less than $25); and an "unlimited" service for checking, debit, and credit-card accounts, priced at $99 annually.

Soon, you'll likely be able to add some benefits of virtual numbers, such as spending limits and merchant restrictions, to regular credit cards and debit cards. MasterCard, which bought the company that licenses the software used for most virtual credit card offerings, says it is working on a service with both features. In the meantime, virtual credit card numbers will keep potential data thieves chasing a moving target.