A background check is a must for potential employees and tenants. However, if you are performing a background check as a landlord or employer, you can't use just any service. The same goes for credit, medical or insurance reasons.
Under the Fair Credit Reporting Act, you have to use a Consumer Reporting Agency. A CRA is federally regulated to maintain high standards for privacy protection and dispute resolution.
If you do reject a potential tenant or employee (even a semi-informal employee like a babysitter) because of a non-CRA background check, you could wind up in legal trouble with the Federal Trade Commission.
The Consumer Finance Protection Bureau has assembled a fairly complete list of CRAs in PDF format. You'll need a program like this free PDF reader to view it, although you may already have one.
The list is helpfully divided into categories such as credit reporting, employment history, insurance, renting and so on. It even has clickable hyperlinks to company websites.
This is also helpful if you want to request and dispute information these CRAs have stored for you.