If you are trying to locate somebody and know what city and county they live in, you can use several forms of public records determine where they live.
The Key Places to look include: 1) County Recorders Office 2) The city/county Treasurer's Office 3) The County Court.
For example, if you are trying to find a person who you know lives (or lived in) Chicago... You can Google, "Chicago County" and find out that it is in Cook County.
Then you find the Cook County Recorders Office (ccrd.info) and see if the individual has ever bought property or had a mortgage in the county. If they currently own property, you can go search the County Treasurer's website to find out where the real estate tax bills are being mailed to---Then you'll be able to find that person's mailing address and get in touch with that long lost relative.
Now, if you're looking for somebody who was never a property owner, then it's more challenging to locate them through public records---You could search the online court cases to see if they have ever been sued or arrested---but odds are pretty good that the addresses in those documents may have gone stale.