edX is a large provider of open online courses. It offers free online university-level courses. edX is a non-profit organization that runs on open-source software.
Youtube is a totally free platform where you can learn anything such as programming, ethical hacking, Graphic designing, etc.
Udemy is an online learning marketplace with around 100000+ courses and 24+ million students who do online learning from udemy. Here some courses are for free and for some courses you have to pay.