The U.S. has a new Passport Card that can be used for land or sea re-entry into the U.S. from Canada and I believe from Caribbean cruises. It costs less than a regular passport. It is $20 if you already have a passport and $45 if you don't. It's the size of a credit card so it's a bit easier to carry, can serve as a spare of sorts and is a bit more durable than a regular passport. It might make a convenient back-up for some. I was planning on getting one but then travel to Juarez hasn't been that safe this year.