Warning: this site has caveats, so read this all! (Note also that this site is pretty much useful only for those living in the U.S., including its possessions.)
ACR is run jointly by the big three U.S. credit report companies (TransUnion, Equifax and Experian). They are now required by law to provide each consumer a free copy of their credit reports on request every 12 months. This site is one way to get that. Why should you bother? Two big reasons: 1) to watch for suspicious activity so you can catch, say, identity thieves more quickly, and 2) to check for errors on your report(s) so you can get them corrected.
By using the web site, you can instantly view all of your credit reports. They verify your identity by asking a number of questions about things they have in your file; it’s reasonably secure, but weird to type in such things as your Visa Card number, mortgage payment, etc. but hey: they already know about that stuff! If it unnerves you, the site also tells you how to request your report copies by phone or mail.
The biggest caveat: they pitch you hard when you use the online service to pay a fee to view your “credit score” and/or to sign up for a “credit monitoring” service. You don’t need to do that to see your basic credit report, so if you’re susceptible to such pitches, don’t do it online, just use phone or mail. But one way or another, you should check your credit reports regularly. It’s a rough world out there, and this is one way to fight back.