No certificates from two of these but they are free if you are into self-passed learning.
Topics can be very complex and a lot of it is open ended
https://ocw.mit.edu/courses/find-by-department/
Here is another great one after making an account they have videos followed by lessons to do. Great for brushing on certain skills like math since they have a lot of problems to solve.
https://www.khanacademy.org/
For one that cost money where you can earn a certifications. https://www.edx.org/ Just keep in mind a lot of the schools patriating in this one intended for people to go off to their college to finish a degree in many cases so I’m not sure how valued a “certification” is. You however can choose to “free audit” a class. Your work just won’t be graded and no certification will be earned. Some of them are opened ended learn at your own pace while others are weekly commitments.
Thanks for the extra reccomendation @enjar
I have heard of Khan Academy and checked out their website once.