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 (own account from age 14+ but younger learners can use a parent account).

Openlearn
Free taster courses aimed at those considering Open University but everyone can access it. Adult level, but some e.g. nature and environment courses could well be of interest to young people.

Ted Ed
All sorts of engaging educational videos

National Geographic Kids
Activities and quizzes for younger kids

Duolingo
Learn languages for free

The Kids Should See This
Wide range of cool educational videos

Crash Course
You Tube videos on many subjects

Crash Course Kids
As above for a younger audience

Big History Project
Aimed at Secondary age, multi disciplinary activities

Geography Games
Geography gaming!

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