Windows Phone Internals is an open-source tool created by a hacker (XDA user) who goes by the name Heathcliff, that performs tasks like unlocking bootloader, enabling root access, and flash or backup firmware.
Heathcliff released the first version after few months of working on it.
Many Lumia phones are supported.
But be careful, this tool is intended only for experienced users and can cause the phone to become unusable permanently.
Github repository: https://github.com/ReneLergner/WPinternals
Official website (dead now): http://www.wpinternals.net/