If you want to learn something new:
Courses on Multiple Subjects
Khan Academy https://www.khanacademy.org/
Saylor https://saylor.org/
Allison https://alison.com/
Udemy https://www.udemy.com/
Coursera https://www.coursera.org/
EdX https://www.edx.org/
Udacity https://www.udacity.com/
Linkedin Learning https://www.linkedin.com/learning/
General Assembly https://generalassemb.ly/
Skillshare https://www.skillshare.com/
Futurelearn https://www.futurelearn.com/courses/upcoming
Academic Earth http://academicearth.org/online-college-courses/
Coding / Tech
Code.org https://code.org/
Codeacademy https://www.codecademy.com/
Pluralsight https://www.pluralsight.com/ https://www.pluralsight.com/codeschool
Adobe Tutorials https://helpx.adobe.com/creative-cloud/tutorials-explore.html
Google Courses https://skillshop.withgoogle.com/
Languages
Memrise https://www.memrise.com/
Duolingo https://www.duolingo.com/
Fluentu https://www.fluentu.com/en/
Cooking
Online Cooking School https://www.onlinecookingschool.com/
The Kitchn https://www.thekitchn.com/collection/cooking-school
Serious Eats https://www.seriouseats.com/
Basics With Babish https://basicswithbabish.co/
Music
Yousician https://yousician.com/
Justin Guitar https://www.justinguitar.com/
Pianonanny https://www.pianonanny.com/index.html
Creative
Masterclass https://www.masterclass.com/
Tests and Quizzes
Fatskills (put your education to the test) https://www.fatskills.com/
Typing
Livechat Typing Speed Test https://zdcs.link/2krgD
10 Fast fingers https://10fastfingers.com/typing-test/english
Typing Test https://www.typingtest.com/
Free Textbooks
CK12 https://www.ck12.org/student/
Openstax https://openstax.org/
Open Textbooks https://open.umn.edu/
Free Kids Textbooks https://freekidsbooks.org/
Bookboon https://bookboon.com/en/textbooks
Internet Archive Books https://archive.org/details/books
Learn by Reading - Project Gutenberg https://www.gutenberg.org/
Also read:
The Fatskills Directory of Online Learning Resources & Tools