It’s handy to be able to check your credit report to ensure that the information companies are reporting about you is accurate. It’s especially important to check before you plan on applying for credit, but errors can sometimes take a while to resolve, so it’s a good plan to check regularly anyway. There are 3 credit reporting bureaus in the U.S.

Federal law (FCRA) allows you to get one report from each bureau for free every 12 months.
Massachusetts law (M.G.L. c.93, § 59(d)) allows you to get one report from each bureau for free within each calendar year. (Some other states have similar laws.)

Note that this combination means that MA residents can get a report of some sort 6 times a year, so they can check their credit for free every 2 months.

If you’re married, your spouse can check his or her credit in the alternating months, for a report of some sort on one of you every month.

For the report via federal law, visit AnnualCreditReport.com. You’ll specify the bureau to get the report from on that site.

Getting the report via state law is trickier, as you need to do it on the individual credit bureau’s web site, and they tend to bury it pretty deep.
For Equifax: http://www.equifax.com/fcra/ and select the reason “Free State Credit File (not denied)”.
For TransUnion (Updated 11/1/2013): https://disclosure.transunion.com/dc/disclosure/disclosure.jsp?. Select “Your state offers a free or reduced price Personal Credit Report”.
For Experian: http://www.experian.com/freestate/

Be sure to read carefully and be vigilant to make sure that you’re only getting the free report, and not signing up for a credit score or monitoring service at the same time. The marketing departments of these companies are very pushy.

As a side note while speaking of credit reports, you may want to sign up at https://www.optoutprescreen.com/ to opt out of pre-approved credit offers.