The U.S. Deptartment of State began production of the "U.S. Passport Card" on July 14, 2008. As of March 2010, more that 2,700,000 Passport Cards have been issued to U.S. citizens.
The passport card can be used to travel to Canada, Mexico, the Caribbean, and Bermuda by land or sea ports-of-entry. This wallet-size card is more convenient and less expensive than a passport book. The passport card however, cannot be used for international travel by air.
All U.S. citizens are eligible to apply for a passport card. The passport card may be requested itself, or together with the passport book. If you have a U.S. passport book and are eligible for a renewal, you may apply for the card by mail using form DS-82. If you never had a U.S. passport book, you must apply in person, using form DS-11.
Passport cards for minors are $15.00 and are valid for 5 years, while cards for adults are $30.00 and valid for 10 years. Otherwise, all requirements are the same as the passport book.