As I mentioned in my intro, I'm a paralegal. One of the things I am often asked to do is locate people. Most of these people are not hiding and are not hard to locate. Their names and phone numbers are in my local White Pages. Other times it is not that easy. Either people are deliberately keeping a low profile or they just don't have listed phones. Our firm subscribes to Accurint, the database that was hacked around Christmas time. During the training session, the sales rep. told us that they buy data from all sorts of places, and that one address they have on his brother is an apartment he (the rep) rented one summer during college, using his brother's name. They had every apartment in which I ever lived, except for the one over some people's garage where I lived for three months right after college. They also provide a list of neighbors' names and phone numbers, as well as a list of names and phone numbers of people they think are related to you. It is kind of scary to think that such information is so easily available, but at least with Accurint, a subscription is necessary, and they only sell to businesses with a legitimate use for the information.
A resource that does much the same thing as Accurint is Knowx. Knowx requires a subscription but sells them to anyone with a credit card. It's prices for searches and records are reasonable, though likely high enough to put off nosy neighbors. Zabasearch allows anyone to enter a name and to retrieve a person's address (with a link to several mapping sites) and month and year of birth. For a fee they will provide a complete background check.