Long Zheng, Rafael Rivera and Chris Walsh, released an elegantly simple ChevronWP7, a Windows Phone 7 unlocker. This tool allow you to deploy (sideload) any third-party WinP7 applications without requiring a Marketplace developer account.
The unlocking process doesn't modify or damage your phone's operating system in any way and can be easily reversed.
Unlocking allows the sideloading of experimental applications that would otherwise can't be published to the Marketplace, such as those which access private or native APIs.
This tool is completely safe and reversible for the phone.
More Info and Download: ChevronWP7