Intellectually Stimulating Web Sites...

Starting with:

Videos and Lectures

• 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

Youtube Channels

• SciShow
• CrashCourse
• MinutePhysics
• TEDEducation

EDIT:
Subreddits

• r/DepthHub
• r/explainlikeimfive
• r/RedditGuides
• r/TrueReddit

Many more listed in the comments...