Credit Sesame offers your Experian score, which is not the same as the more recognized FICO score but does give you a pretty good indication of where your FICO score is (FICO's model is proprietary.). The inquiry is a "soft pull," which means that it does not show up on your credit report and will not affect your credit.
Why is it free? Because they'll try to sell you extras, of course. Unless you have a burning desire for their other services, you can ignore them.
Bottom line: It's good to have a handle on where your credit score is and why it's there, and CS is a service that will get you most of the way for free. Note that if you're actually applying for a mortgage, loan, etc., you may be better off paying for the FICO score. But if you just want to keep an eye on your score, look for trends as you apply for credit cards, etc., Credit Sesame is a good place to start.
Check out the site here. It may take a second to load.
Note: CS does claim to offer me a small (very small) amount of money if you sign up using this link. It is much appreciated. Will they actually pay me something? Who knows.