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.
Probably Khan Academy
Wow this is great
Here is a list of websites I had saved from Reddit about 8 years ago. Apologies in advance, I haven't checked any of the URLs if they are still valid.
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/I know you asked for one, but I'm going to list a whole bunch of websites that I use/used during school. Bolded are my favorites from each category.
General Websites
Math/Science
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.
http://tug.ctan.org/info/undergradmath/undergradmath.pdf LaTeX math symbols
LaTeX online editor. I use this when my LaTeX isn't working, or if I'm using a new operators I haven't used before
https://guides.nyu.edu/LaTeX/creating-document Guide by NYU for creating LaTeX documents
https://math.berkeley.edu/~ehallman/math1B/ Worksheet/Quizzes from a UC Berkeley calc 2 class
Paul's Online Notes. This link is specifically for calc 2, but he has notes from algebra all the way to diff eq.
https://www.youtube.com/@patrickjmt/featured Great channel for math
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.youtube.com/@TheOrganicChemistryTutor Great channel for math, chemistry, and physics. I use him more for examples rather than learning.
https://pubchem.ncbi.nlm.nih.gov/periodic-table/ Periodic table from PubChem. This is also a stand in for the websites of PubChem and PubHealth
French
I'm learning French currently and I have a few resources other than YouTube that is helpful
https://en.wikipedia.org/wiki/Help:IPA/French This is the French IPA. Very useful to learn
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.laits.utexas.edu/tex/gr/index.html French grammar from the University of Texas
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.
deepl.com Translator
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
some big tech companies have courses on their websites, cisco microsoft google etc, worth to check imo.
Coursera
