Credit to /u/Fletch71011
Education.
No Excuse List - Includes sources for everything you can want. I included some more popular ones with brief write-ups below. Credit to /u/lix2333.
Reddit Resources - Reddit's List of the best online education sources
Khan Academy - Educational organization and a website created by Bangladeshi-American educator Salman Khan, a graduate of MIT and Harvard Business School. The website supplies a free online collection of micro lectures stored on YouTube teaching mathematics, history, healthcare and medicine, finance, physics, chemistry, biology, astronomy, economics, cosmology, organic chemistry, American civics, art history, macroeconomics, microeconomics, and computer science.
Ted Talks - Talks that address a wide range of topics ("ideas worth spreading") within the research and practice of science and culture, often through storytelling. Many famous academics have given talks, and they are usually short and easy to digest.
Coursera - Coursera partners with various universities and makes a few of their courses available online free for a large audience. Founded by computer science professors, so again a heavy CS emphasis.
Wolfram Alpha - Online service that answers factual queries directly by computing the answer from structured data, rather than providing a list of documents or web pages that might contain the answer as a search engine might. Unbelievable what this thing can compute; you can ask it near anything and find an answer.
Udacity - Outgrowth of free computer science classes offered in 2011 through Stanford University. Plans to offer more, but concentrated on computer science for now.
MIT OpenCourseWare - Initiative of the Massachusetts Institute of Technology to put all of the educational materials from its undergraduate- and graduate-level courses online, partly free and openly available to anyone, anywhere.
Open Yale Courses - Provides free and open access to a selection of introductory courses taught by distinguished teachers and scholars at Yale University.
Codecademy - Online interactive platform that offers free coding classes in programming languages like Python, JavaScript, and Ruby, as well as markup languages including HTML and CSS. Gives your points and "level ups" like a video game, which is why I enjoyed doing classes here. Not lecture-oriented either; usually just jump right into coding, which works best for those that have trouble paying attention.
Team Treehouse - Alternative to Codecademy which has video tutorials. EDIT: Been brought to my attention that Team Treehouse is not free, but I included it due to many comments. Nick Pettit, teaching team lead at Treehouse, created a 50% off discount code for redditors. Simply use 'REDDIT50'. Karma goes to Mr. Pettit if you enjoyed or used this.
Think Tutorial - Database of simple, easy to follow tutorials covering all aspects of popular computing. Includes lots of easier, basic tasks for your every day questions or new users.
Duolingo - For all of your language learning needs.
Memrise - Online learning tool that uses flashcards augmented with mnemonics—partly gathered through crowdsourcing—and the spacing effect to boost the speed and ease of learning. Several languages available to learn.
Livemocha - Commercial online language learning community boasting 12 million members which provides instructional materials in 38 languages and a platform for speakers to interact with and help each other learn new languages.
edX - Massive open online course platform founded by Massachusetts Institute of Technology and Harvard University to offer online university-level courses in a wide range of disciplines to a worldwide audience at no charge. Many other universities now take part in it, including Cal Berkeley. Differs from most of these by including "due dates" with assignments and grades.
Education portal - Free courses which allow you to pass exams to earn real college credit.
uReddit - Made by Redditors for other Redditors. Tons of different topics, varying from things like science and art to Starcraft strategy.
iTunes U - Podcasts from a variety of places including universities and colleges on various subjects.
Stack Exchange - Group of question and answer websites on topics in many different fields, each website covering a specific topic, where questions, answers, and users are subject to a reputation award process. Stack Overflow is used for programming, probably their most famous topic. Self-moderated with reputation similar to Reddit.
Wikipedia - Collaboratively edited, multilingual, free Internet encyclopedia. Much better source than most people give it credit for, and great for random learning whenever you need it. For those looking for more legit sources for papers and such, it is usually easy to jump to a Wikipedia page and grab some sources at the bottom.
Edit: Thank you for the gold /u/NekoGamiYuki although I don't deserve it.