Carrier policies for unlocking devices

Many people will be selling their current Android or iPhone device to make room (and cash) for the new iPhone 6 and iPhone 6 Plus. An unlocked unit will earn more money on sites like Gazelle that pay cash for your old phone or in the private market.

Here are links to the Device Unlock Policy for each major US carrier.

AT&T: click here.
T-Mobile: click here.
Sprint: click here.
Verizon: click here. (Most Verizon LTE devices come unlocked.)
O2 (UK Carrier): click here.
Virgin Mobile (USA): click here.