While Apple has only offered the iPhone 6 and 6 Plus on contract since its launch (with the exception of the T-Mobile variant), it is reportedly set to start offering the devices unlocked and SIM-free starting tomorrow, January 6. According to a report by 9to5Mac:
Apple will start selling fully unlocked, SIM-free iPhone 6 and iPhone 6 Plus models in the United States beginning tomorrow, January 6th, according to several sources. The unlocked iPhones will be available both via Apple's physical retail stores and online store
9to5Mac also notes that the prices will remain the same as the current unlocked T-Mobile variant of each device.