Among the manufacturers that will allow you to unlock the bootloader on your Android phone is Sony. Back when the company was known as Sony Ericsson, this would have seemed unfathomable. Now, Sony is actually posting a video on YouTube, showing Xperia owners how they can unlock their device.
In the video, the Sony Xperia Z2 is being used as an example. A Windows computer is also employed. As you can see, it basically is a matter of following the steps that are fairly easy to understand. Keep in mind that even though Sony seems to condone unlocking the bootloader by posting this video, if you go ahead and do it to your phone, you will lose the warranty on the device. Even worse, if you don't know what you are doing, you could brick your phone.
source: SonyXperiaDev via XDA