Art and Music
Dave Conservatoire — Dave Conservatoire is an entirely free online music school offering a self-proclaimed “world-class music education for everyone,” and providing video lessons and practice tests.
Drawspace — If you want to learn to draw or improve your technique, Drawspace has free and paid self-study as well as interactive, instructor-led lessons.
Justin Guitar — The Justin Guitar site boasts over 800 free guitar lessons which cover transcribing, scales, arpeggios, ear training, chords, recording tech and guitar gear, and also offers a variety of premium paid mobile apps and content (books/ ebooks, DVDs, downloads).
Math, Data Science and Engineering
Codecademy — Codecademy offers data science and software programming (mostly Web-related) courses for various ages groups, with an in-browser coding console for some offerings.
Stanford Engineering Everywhere — SEE/ Stanford Engineering Everywhere houses engineering (software and otherwise) classes that are free to students and educators, with materials that include course syllabi, lecture videos, homework, exams and more.
Big Data University — Big Data University covers Big Data analysis and data science via free and paid courses developed by teachers and professionals.
Better Explained — BetterExplained offers a big-picture-first approach to learning mathematics — often with visual explanations — whether for high school algebra or college-level calculus, statistics and other related topics.