They're not all scams. Anyone read EGM? They did an article on these types of sites and they found that they are often legit.
Here's how it works:
You have to sign up for a lot of offers from other companies (online movie rentals, music download sites, etc.) to get the free product. These offers usually require you to enter your credit card number, and then they start you on a free trail period of that offer. After the trial period ends, the company holding the offer will start charging to your credit card, therefore, you should call all the company a couple of days before the trial period is over to cancel what you signed up for, so no money is charged.
You have to do this with a lot of offers, and it's a time consuming process to stay on top of everything, but if you're willing to devote your time, you should get the free product you wanted in the mail.