Due to the passage of the 2003 Fair and Accurate Credit Transaction Act (FACTA), all Americans are entitled to one free credit report from each of the three major credit reporting agencies -- Equifax, Experian and TransUnion -- upon request every 12 months. You can do this easily at http://www.annualcreditreport.com.
This is the only one that is TRULY free -- don't let the others fool you, they will probably want you to sign up for a free trial of their services. If you are interested in your credit score, I recommend CreditKarma.com. They are also free. Avoid having everyone you see pull your credit score for you - it will effect your credit score!