You'll get free products before they're released. Usually, a public beta is a pre-release product available for download or sent via CD to listed testers.
Private betas are a little bit harder to get, of course.
Microsoft will regularly let potential testers sign up to test its private betas, unlike many other companies. Of course, Microsoft runs the most popular private beta programs. However, Microsoft already has a list of testers to whom they send their betas. If you are not affiliated with Microsoft, getting a beta may not be as easy as it sounds. First of all, ask people you know if they're on the list.