But they're GSM, so you can't use them on any network you want.
When the first iPhone launched, it was only available on AT&T, and it was locked to that carrier. In fact, unless you lived in a country where consumer laws prevented Apple from locking its phone to one carrier, you didn't have a whole lot of choice when it came to carrier no matter where you lived. In the years since, folks have gained a little more freedom when it comes to the iPhone. The device is now available across multiple carriers and, as of today, you can now get the phone unlocked and SIM-free through Apple.
Apple is now selling a GSM version of the iPhone 5S that is SIM-free and not attached to any carrier. This version will cost you $650, $750, or $850 depending on the capacity you're after (16 GB, 32 GB, and 64 GB, respectively), and is currently shipping in 1-2 weeks. You could also opt for the iPhone 5C, which is a little bit cheaper and is also available unlocked and SIM-free. That'll set you back $549 for the 16 GB model or $649 for the 32 GB model.
The iPhone 5S was launched in September and packs Apple's 64-bit A7 SoC, a Retina display, TouchID authentication on the home button, and iOS 7. The iPhone 5C runs on Apple's A6 SoC and features a 4-inch retina display, an 8-megapixel iSight camera, a new Facetime HD camera with improved low-light functionality, dual-band WiFi a/b/g/n, Bluetooth 4.0, more LTE bands than any other phone, and is available in pink, yellow, green, blue and white.