Windows Phone 7 was released in October 2010 and now we have the first unlocking tool. This unlocking tool, ChevronWP7, allows you to run experimental apps without the need for a marketplace account. The unlocked is developed by Rafael Rivera, Long Zheng and Chris Walsh. You can now run apps which are not listed in Marketplace and also such as those which access private or native APIs.
This tool is completely safe and reversible for the phone. (The app even allows you to relock the phone.)
To unlock the phone, you need to connect the phone to your PC using the USB cable and then run the Zune software. Now run the exe file and you can unlock your Windows Phone 7.