@openculture: Tons of courses, ebooks, audio books. Great place to start if you want to start learning a language.
@coursera: Courses provided by universities and organizations. Not as free as it used to be.
@MITOCW: MIT video lectures.
@gutenberg_org: 60,000+ free ebooks. Great place to get the classics.
@memrise: See above. Especially good if flash cards are your preferred vocabulary study aid.
@unsplash: Hundreds of thousands of stock images.
@internetarchive: Books, music, software, games, etc.
Feel free to add more, folks. Never stop learning!
@periodicvideos: Informative, awesome videos about chemistry that cover each element.
@numberphile: See above, but with math.
serc.carleton.edu/teachearth/ind…: Teach The Earth a resource index for anything Earth Science/Geoscience.
mw.concord.org/modeler/index.…: Molecular Workbench does simulations involving physics, chemistry, biology, etc.
nature.com/scitable/: Resources for biology and genetics.
@Yale's Open Courses at oyc.yale.edu/courses: Video lectures on English, sciences, economics, history, sociology, etc.