All courses found on our Lagunita courses page here, https://lagunita.stanford.edu/courses, are offered for free. However, some of the courses may recommend materials for the course, at a charge. Typically, these fees are optional and not required.
Online courses on some of Stanford's other platforms may not be free. To find out if they are or not, go to the platform in question and click to sign up or register. In most cases, information will be provided on a course's specific enrollment page (if not on the listing page directly).
For example, at online.stanford.edu/courses/platform/NovoEd, you can click on a particular course's name. This will take you to a page with more information about the course. From the page you are taken to, you will see if there is a fee for the course, or if it is free.
For the Stanford Center for Professional Development (SCPD) courses at scpd.stanford.edu, fees may be noted under a Tuition section or in FAQs for specific courses.