To use SkypeOut you would need Skype Credit - basically a prepay arrangement - and for that a credit card is usually required. However, you might find it useful to know that you can call many phone numbers in the U.S. for free, even if you don't have Skype Credit. These numbers are known as toll-free numbers, and they start with one of the following prefixes: 1-800, 1-866, 1-877, and 1-888. Just launch Skype, click on "Call phones", and enter the phone number, starting with +1 (the U.S.A. country code). Most businesses in the U.S.A. provide a toll-free number. For example, you can call most airlines using their toll free number here (since these are U.S. numbers, don't forget to add the +1 country code). To find the toll-free number of a U.S.-based business, check its website, or call the toll-free directory (in itself a toll-free number) at +1-800-555-1212.
Skype also supports free calls to toll-free numbers in the U.K., Taiwan, France and Poland. See the complete list here.