Which student resource sites are known for curriculum-aligned lessons and easy-to-follow explanations?

Some widely used student resource sites known for curriculum-aligned lessons and easy-to-follow explanations include:

• Khan Academy — clear instructional videos and practice across math, science, humanities, and test prep.
• BBC Bitesize — especially strong for UK curriculum support with concise explanations and revision materials.
• CK-12 — free lessons and practice in math and science, often aligned to school standards.
• iCivics — curriculum-friendly civics and government lessons explained in student-friendly language.
• National Geographic Education — approachable lessons and resources for geography, science, and social studies.
• PBS LearningMedia — classroom-aligned videos, interactives, and lesson resources for many subjects.
• Quizlet — not a full lesson site, but useful for curriculum-based study sets, flashcards, and explanations.
• Study.com — structured lessons and tutorials aligned to many school and test-prep topics.

If you want, I can also narrow these down by grade level, country/curriculum, or subject.