Where is that phone number from? Everybody knows that the US and Canada are divided up into Area Codes, three-digit numbers that help telephone switching equipment know where you are calling when you dial a number: 212 for Manhattan, 213 for L.A., and so forth. Within each area code are a number of "exchanges" that serve to further localize a given phone number. Put them all together, and you get NPA-Nxx (Numbering Plan Area – Central Office) system. With these pieces of a phone number, like 212-222-xxxx, you can focus in on the neighborhood that a phone number belongs to.
The NPA-NXX Geolocator is a phone prefix lookup tool that uses Google Maps to help show you where a particular phone number belongs. Type in the area code and prefix, hit the Search button, and you're whisked to a map of the location. Obviously you're not going to get an arrow pointing to a particular address—how would you dial "xxxx" anyway?—but it's going to put you on the map in terms of the area where the number belongs.
The information displayed here all comes from publicly-available sources, and there is a disclaimer about not using it for any kind of mission-critical applications.
Thanks to Mitch for pointing this one out to us.