What are the most intellectually stimulating websites of which you know?

What about them is intellectually stimulating?

Answer Wiki

Tuhin KumarTuhin Kumar, Facebook, Design, Startups, Mu... (more)
214 upvotes by Amit Jain, Rick Stevens, David J. Cohen, (more)
TED: http://ted.com
Bret Victor: http://worrydream.com
Craig Mod: http://craigmod.com
Bobulate: http://bobulate.com/
Frank Chimero: http://frankchimero.com/writing/
xkcd (totally serious) - http://xkcd.com/
McSweeny's: http://www.mcsweeneys.net/
Paul Graham's Essays: http://paulgraham.com/articles.html
What If XKCD: http://whatif.xkcd.com/
Explore: http://exp.lore.com/
Brain Pickings: http://www.brainpickings.org/
I.M.H.O on Medium: https://medium.com/c/2ed1867362f8

(Will keep adding if anything else crosses my mind)
  
Abhay BothraAbhay Bothra, On Quora for this...
Brainpickings (http://www.brainpickings.org/) by Maria Popova is by far one of the most intellectually stimulating sites I have come across.
  
Salim HuertaSalim Huerta, Future doctor and entrepreneur
226 upvotes by Mike Townsend, Quora User, Rushabh Kamdar, (more)
These are all the sites named so far in case you just wanted to copy and paste them all really quickly. You're welcome

 
KhanAcademy.org
TED.com
UnplugTheTV.com
Learn New Things
FuckingHomepage.com
IfEveryoneKnew.com
Testing Knowledge
FreeRice.com
LizardPoint.com
MathRun.net
Teach Yourself Something
OpenCulture.com
NoExcuseList.com
http://udacity.com/ - Free computer science lessons.
Project Euler is good if you posses some programming skill and an interest in math. http://projecteuler.net/
YouTube Channels
SciShow
CrashCourse
MinutePhysics
TEDEducation
Quora
Wikipedia
TED: http://ted.com
Bret Victor: http://worrydream.com
Craig Mod: http://craigmod.com
Bobulate: http://bobulate.com/
Frank Chimero: http://frankchimero.com/writing/
xkcd (totally serious) - http://xkcd.com/
McSweeny's: http://www.mcsweeneys.net/
Paul Graham's Essays: http://paulgraham.com/articles.html
What If XKCD: http://whatif.xkcd.com/
Explore: http://exp.lore.com/
Brain Pickings: http://www.brainpickings.org/
I.M.H.O on Medium: https://medium.com/c/2ed1867362f8
artsandlettersdaily.com
Brainpickings (http://www.brainpickings.org/)
http://www.edge.org/ - my absolute favorite
http://www.3quarksdaily.com/ - to certain extent.
Stanford Encyclopedia of Philosophy: http://plato.stanford.edu/
On rationality:
http://www.overcomingbias.com/
http://lesswrong.com/
StackOverflow
Codecademy
Udacity
Coursera
http://bigthink.com/
http://www.mentalfloss.com/ - where knowledge junkies get their fix
http://hypertextbook.com/
http://www.metafilter.com/ for general knowledge
http://www.aldaily.com/ for long reads more related to the arts
http://www.themorningnews.org/ for interesting links and quirky long reads
http://www.reddit.com/r/bestof/ for great writing
http://www.volokh.com/      Volokh Conspiracy: Famous blog by several lawyers, most with a conservative/libertarian bent.
http://cscs.umich.edu/~crshalizi...      Three-Toed Sloth: terrific and not-too-irregularly updated blog by statistician, complexity theorist and all-around intellectual Cosma Shalizi, professor at Carnegie-mellon, capturing his thoughts on pretty much anything in the realm of science, political philosophy and literature.
http://youarenotsosmart.com/
http://www.PandaWhale.com - Reddit + Digg
http://moreintelligentlife.com - The Economist's coolest editorials
https://teamtreehouse.com - Learn web tools
http://facingthesingularity.com - Get lost in Singularity
https://freenetproject.org - Private and Anonymous  
http://openmymind.net - Random Blog
http://cyborganthropology.com - Traditional vs. Cyborg Anthropology
http://www.radiolab.org/
MIT open courseware - http://ocw.mit.edu/index.htm
Hackthissite.org
Hackaday.com
Lifehacker.com
Wilywalnut.com
W3schools.com
Lumosity
lurnQ
Lifeedited.org
http://blogs.discovermagazine.co...
http://www.jamesaltucher.com/
 
http://thoughtcatalog.com/
This one is a good one: specially for those that are beggining with Python programming.
http://www.pythonchallenge.com/
http://crookedtimber.org/
http://www.thebrowser.com/
google
http://igniteshow.com/
Hackerne.ws
http://michaelchichi.com/stimuli...
http://zombo.com/
How Stuff Works
Cracked.com
xkcd
Dilbert Blog
mashable.co
gsmarena.com
cnet.com
yahoo answers (Just like Quora)
  
Sara ZasowskiSara Zasowski, Servant to the Internet
38 upvotes by Anna Demers, David J. Cohen, Quora User, (more)
Arts & Letters Daily always has a great mix of new information, articles, essays, news, etc.

artsandlettersdaily.com
  
Quora UserQuora User, eternal beginner
On rationality:
Both websites are focused on analyzing situations more rationally and (for me) often point out biases and problems in my thinking that I had been completely unaware of.
  
Patrick CollisonPatrick Collison, Books, code, tools, waffles.
47 upvotes by Quora User, Ethan Herdrick, Aline Lerner, (more)
SoleioSoleio, Dropbox
82 upvotes by David J. Cohen, Quora User, Joshua Blankenship, (more)
Wikipedia is an obvious answer.
  
Avinash SMAvinash SM, Indian | भारतीय | ਪੰਜਾਬੀ | h4ck3r
32 upvotes by Rohit Shinde, Quora User, Quora User, (more)
  1. You’re getting old – Website that gives you stats about your life so far in an awesome way.
  2. Clickhole – BuzzFeed parody from the creators of The Onion sarcastic news site.
  3. Made /w Code – A customized bracelet that you can have 3D-printed. Made by Google.  Also they will send you it for free if you live in the US.
  4. Printable Paper – Useful for school start and saves you a lot of money. Download and  print many kind of papers for free like “Dot Paper with 2.5mm spacing on  A4-sized paper” or “Columnar Paper with six columns on A4-sized paper  in landscape orientation” and hundreds more.
  5. Joshworth.com – If the Moon were only 1 pixel. Scroll and be amazed!
  6. ChessCademy – Awesome site to learn chess.
  7. Zanorg.com Blog – French blog, but the Flash application is unbelievably cool. 40  popular songs split up into the used instruments, each instrument can be  activated and deactivated separately.
  8. Gigapan.com – 45 gigapixel panorama picture of Dubai. Zoom!
  9. Mailinator – Disposable e-mail account. Useful when you don’t want to receive spam to your main e-mail account.
  10. Midomi – The site is like Shazam, but for PC, however you can sing the song yourself or hum it if you don’t know the words.