Number Guru is a reverse phone number search engine for your Windows Phone. Described as the "biggest, baddest, and most handsome callerID and reverse phone service you've ever seen", the app claims to cover about 100% of the landlines and 50% of cell phones (excluding Verizon numbers). Not sure about the "biggest, baddest, and handsome" claim but Number Guru does cover a lot of territory by tapping into 334 area codes in the U.S. and U.S. territories (including the 1-888 and 1-800 numbers).
So now you have a means to reverse look-up those mysterious numbers that appear on your Windows Phone. The app searches are free and unlimited. You also have the ability to add comments to the phone numbers to help other users identify the type of caller (e.g. telemarketer).
Number Guru also maintains a history of numbers searched for quick reference. The interface is independent of your Windows Phone in that you have to enter the number manually (the ID won't pull up when the phone rings).
Number Guru is a free app that can be downloaded here (opens Zune) at the Marketplace.