Harvard's introduction to computer science, free to audit online.

Project-based web development curriculum with free certifications.

Free lessons and practice across maths, science and economics.

Visual step-by-step learning paths for developer and data roles.

Authoritative reference for HTML, CSS, JavaScript and browser APIs.

Dense one-page cheat sheets for tools, languages and command lines.

Course materials, lecture notes and problem sets from MIT courses.

Free full-stack curriculum built around building real projects.