The other day, I was thinking about adding a few certificates to my resume and linkedin profile without spending a fortune, and my quest began with a simple Google search like "free online computer courses". Believe it or not but the search results for this query are highly misleading because none of the courses offered by popular providers like Coursera, edX, Udemy, etc. are free in the true sense. At most, you may "audit" or access the course materials for free but when it comes to earning a certificate that says you really underwent that course, you're faced with a giant paywall!
Not being among those who give up a quest too easily, I kept the pursuit on and found at least four such truly socialist course providers who still provide online education for zero fees without any commercial interests.
Now, some of these providers may ask you for a voluntary donation (FreeCodeCamp) or have optional premium content (SoloLearn) to keep their services running but as far as I know, none of them charge any money for giving certificates for any of their courses.
Saylor Academy is Australia based and their content is professional and top notch. They provide all kinds of courses on varied topics such as Introduction to Financial Accounting and Introduction to Python. Their courses are in long content format and you can continue them at your own pace.
SoloLearn courses have shorter content and they focus more on quiz and "match the following" style gamified content. They are still quite enjoyable and educational material though.
Full Stack Open initiative is from University of Helsinki, Finland and covers various aspects of full stack web development like react, redux, node.js, mongodb, etc. A nice article written by Laura Ojala outlines several details about this course.
Different people may have different ethical constraints about this but personally I don't think education is something that should be commoditized or commercialized with paywalls. If you happen to know some other course provider who doesn't charge for certificates, let me know in comments and I shall add them to the list.