You can find free online certificate classes by searching directly from the online course sections for major universities such as Harvard, Yale or the University of Washington. Some classes may include a fee for processing the certificate.
Many universities and colleges offer online courses that are open to the public rather than only to enrolled students. Each school manages these programs differently, with some requiring a strict registration process and others only instructing students to create accounts with the site. Many of these courses include fees, either for enrollment or materials, so you must search through all listings to locate totally free courses. Online descriptions for courses usually include details about term duration, concepts covered and the type of certification available upon completion.
Online education portal edX.org also allows you to access various free online courses from universities and other institutions, with a special section of its site housing programs that include a verified certificate. While these courses typically do not include any fees for registration, they often do charge students for processing the official certificate. In some circumstances, it is possible to obtain discounts or waivers for these fees, but you need to work with the institution to obtain such benefits.