Crossposted from one of the top answers from here before (all 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.