As of 2015, to get the newest iPhone for free, sign up with a carrier's monthly plan or trade your older iPhone device to use some of the value towards purchasing a new iPhone. The exact method for obtaining the newest iPhone depends on the carrier and phone model.
The monthly payment option allows you to avoid paying anything at the time of purchase. Instead, the carrier places your payment on your monthly cellphone bill. If you are trading in your existing phone, check the carrier's website to obtain a credit offer. You can offset the cost of the new device depending on the value of your current cellphone.