Khan Academy
Especially good for math and computing for all ages but other subjects at Secondary level. Note this uses the U.S. grade system, but it’s mostly common material.
Futurelearn
Free to access 100s of courses, only pay to upgrade if you need a certificate in your name (accounts require users to be at least 14, but younger learners can use with a parent).
Openlearn
Free taster courses aimed at those considering Open University, but everyone can access it. Developed primarily for adults, but some courses, e.g., nature and environment, could well be of interest to young people.
Blockly
Learn computer programming skills – fun and free.
Scratch
Creative computer programming
Ted Ed
All sorts of engaging educational videos
National Geographic Kids
Activities and quizzes for younger kids
Duolingo
Learn languages for free
Mystery Science
Free science lessons
The Kids Should See This
Wide range of cool educational videos
Crash Course
YouTube videos on many subjects
Crash Course Kids
As above for a younger audience
Crest Awards
Science awards you can complete from home
Tinkercad
All kinds of making
Prodigy Math
Good for elementary school ages
Big History Project
Aimed at Secondary age, multi-disciplinary activities
Geography Games
Geography gaming!
STEM Simulators
Learn Computer Science
The Artful Parent
Good, free art activities linked to from this Facebook page
Red Ted Art
Easy arts and crafts for little ones
The Imagination Tree
Creative art and craft activities for the very youngest
Toy Theater
Educational online games