Lame answer but YouTube. Channels in the vein of freecodecamp.org and Stanford Online are incredible. Any skill, hobby, repair, or question I can think of probably has videos there.
hitching a ride to add on various study helps. free online education dump incoming:
http://education-portal.com/academy/course/index.html
http://101science.com/
https://iversity.org/
http://www.openculture.com/freeonlinecourses
https://www.coursera.org/
https://www.edx.org/course-list
http://www.dliflc.edu/products.html use the GLOSS link
http://www.coursehero.com/subjects/
http://oli.cmu.edu/
http://ocw.mit.edu/courses/find-by-topic/
http://www.saylor.org/
http://ocw.jhsph.edu/
http://www.open.edu/openlearn/
http://ocw.tufts.edu/
https://itunes.stanford.edu/content/rss.html
http://webcast.berkeley.edu/#
http://www.makeuseof.com/tag/3-websites-started-learning-programming-language/
https://www.futurelearn.com/
http://www.flashcardmachine.com/ You can use flashcards made by other users. Whether you trust them is up to you
http://freerice.com/category It quizzes you on the basics of a subject o your choosing, and donates rice for each answer you get right once you turn off adblock
http://openstaxcollege.org/
http://justenglish.me/2012/09/01/free-books-100-legal-sites-to-download-literature/
http://blog.boundless.com/2013/04/the-cost-of-textbooks-is-too-damn-high-so-boundless-made-free-ones/
http://freescience.info/index.php
To the best of my knowledge, these are all free and legal, and of varying degrees of usefulness. here's the thread I originally put it in, which may have some similar stuff. http://www.reddit.com/r/AskReddit/comments/268a0s/what_random_things_can_i_get_certified_for_over/
http://www.foundalis.com/lan/hw/grkhandw.htm Gives instructions on how to write Greek letters. Idk if this is useful for anyone else, but for some reason I have a hard time writing some Greek letters without this.
https://www.youtube.com/@ProfessorLeonard Another great channel for math. He's a professor who uploads his lectures from pre-calc up to diff eq I believe. Videos are long, but well worth it. He's the one of only reasons why I passed calc 1 and why (fingers crossed) I'll pass calc 2 this summer.
https://www.ipachart.com/ How various IPA letters sounds like, not specific to French. They don't have all of the French letters, but useful regardless
phraseFantasic Resource for French language learning. Lots of info on here and while specific for French, can be useful for getting ideas on what/how to learn for other languages
https://www.wordreference.com/ French-English dictionary (not only has more languages than just French and English). Gives context, has conjugations, and has a forum so if your specific usage of the word isn't in the dictionary, you can ask/search the forum.
That's it! There are some websites/extensions that I use for school that I didn't mention here because it's too specific, but here is (mostly) everything that I use regularly during high school and college