World-travelers and those who don't want to tie themselves to a carrier contract can now purchase a carrier-unlocked iPhone 5s, without a SIM card, through Apple's online store.

As with previous unlocked iPhones, the unlocked iPhone 5s costs $649 for a 16GB model, $749 for 32GB, and $849 for 64GB. All three colors--space gray, gold, and silver--are available and have the same one- to two-week shipping window. As previously, you can also order an unlocked iPhone with a T-Mobile SIM card, which will ship in three to five business days.

The unlocked iPhone 5c has been available since launch, and ships within 24 hours, with a price of $549 for 16GB and $649 for 32GB.

An unlocked iPhone could be useful if you spend a lot of time overseas. In many places, you can simply pay month-to-month by picking a local SIM card from whatever country you're in. The rates often end up being much lower than the premium overseas rates charged by U.S. carriers.

For most people, it makes more sense to go to a wireless carrier and get the iPhone at a subsidized price, in exchange for a two-year contract. If you're planning an overseas trip, Verizon, Sprint and T-Mobile will unlock your phone as long as your account has been in good standing for 60 days. AT&T, however, will only unlock the iPhone after your contract is up, though some exceptions to the rule have been reported.

Keep in mind that the unlocked iPhone--which is model A1533 (GSM)--will only work on AT&T's and T-Mobile's networks in the United States. If you need coverage on Verizon or Sprint, you'll have to buy the carrier-locked version, and request an unlock later.