Online free resources to support or replace textbook reading

Math courses:
Calculus I, Calculus II, Calculus III, Differential Equations (website) (full lessons)

Civil engineering courses:
Statics, Strengths of Materials/Mechanics of Materials (YouTube channel) (full lessons)
Structural Loads & Analysis (website) (full lessons with short quizzes to check comprehension)
Civil Engineering Materials (YouTube channel) (source of material testing demonstrations shown in some lectures)

Engineering:
Various engineering disciplines (collection of open-access textbook links) (not experienced with this one)