edX

Founded by Harvard and MIT, edX is a global nonprofit platform offering university-level courses, MicroMasters programs, Professional Certificates, and even online Master's degrees. It emphasizes academic rigor and accessibility, with many free-to-audit courses. Key features include discussion forums, graded problem sets, and verified certificates for a fee.

It targets students and professionals seeking credible, university-backed credentials, particularly in STEM and business. Its nonprofit status and focus on high-quality, transferable academic credit give it strong authority and value, though its interface can be less polished than some competitors.