A selection of YouTube channels that offer relevant content for language, math, science and history learners.
Periodic Table of Videos
It's Okay to Be Smart (PBS)
Minute Physics - Simply put: cool physics and other sweet science.
Veritasium - The science video blog from atoms to astrophysics!
French Help Whenever
CGP Grey - Complex Things Explained.
Vsauce - Strange questions, amazing answers!
Khan Academy - Math, Chemistry, Physics, U.S. History and several other topics
Merriam-Webster Online - regrettably, it would seem that production has stopped several months ago, but the content already present is very relevant.
Crash Course - Literature, Science, U.S. History, Biology