Science Education

• APS Physics Outreach Guide
• NASA Education and Public Outreach
• TED Talks
• PhET: Interactive science applets
• Physlets: Interactive physics applets
• List of Physics Demonstrations
• Physics Demonstration Videos
• Physics For Middle School Education
  
• Map of Every Space Mission
• 100,000 Stars: A simulation of the nearest 100,000 starts in the Milky Way

Physics

• APOD: Astronomy Picture of the Day
• Hyperphysics: Everything an undergraduate should know
• ArXiv: Physics preprint server
• The SAO/NASA Astrophysics Data System: Journal articles database
• Xenon Dark Matter Project: Links to many Dark matter review articles
• A Collection of Astrophysics-based Review Articles
• A Heroic Collection of Dark Matter Papers
• More Dark Matter Papers
• A Collection of  Physics Links
• Another Collection of Physics Links
• Physics Database: eBooks and lectures
  

Mathematics

• Wolfram Alpha: A "computational knowledge engine"
• Wolfram Mathworld
• Math Is Fun
• Ponder This: Monthly puzzles from IBM
  
• Mathematica Tutorial Collection
  
• Texify: Convert TeX to math equations
• archery: Math blog with interesting Mathematica code

Computer Programming 

• Code Academy: Lessons in JavaScript, Python, HTML, PHP, CSS, Ruby and more
• JSFIDDLE: Browser-based compiler for testing JavaScript, HTML, and CSS
• Crunchzilla: JavaScript learning environment for kids and adults
• cplusplus.com (C++)
  
• Scratch: Programming for kids
• Resources to teach programming to kids
• OverAPI: Collection of coding cheat sheets
• Computer Science Circles: Python basics
• Making Games: Blog about making computer games
• Bento: "Everything you need to know about web development. Neatly packaged."
• Programming MF: A vulgar but thorough coding resource 
  
• W3 Schools: We development resource
• Code Avengers: "Learn how to code games, apps, and websites"
• Stack Overflow: Where all your programming questions are answered
• Github: Where to go to share and collaborate on code
• inLook's HTML Reference Page (Thank you Mrs. Lowe's students!)

Data Science/Analytics

• Kaggle:  Big data competitions for big prizes

Electronics/Arduino/Raspberry Pi

• Arduino.cc 
• Arduino Playground: Project ideas
• Best Arduino Projects of 2012: Make Magazine
• Make Arduino Projects
• Adafruit: Parts
• SparkFun: Parts and tutorials
• Mouser Electronics: Parts
• Digi-Key: Parts
• Arrow: Parts

Online Classes

• Academic Earth 
• Khan Academy
• Coursera
• edx
• Yale Open Courses
• MIT Open Courseware
• Udacity

Free Books

• Open Library
• Project Gutenberg 
• ManyBooks.Net
• Planet eBook
• The Internet Archive: Free books, video, audio, etc.
  
• NASA eBooks
• Munseys

DIY/Maker

• Make Magazine
  
• Make Use Of
• instructibles.com
• Guitar Extended: Digital effects and processing DIY 
• LifeHacker
• Hack A Day

Miscellaneous

• HackerThings: A place to buy fun stuff
  
• Useful Websites via TED.com
  
• Alternative Ways to Tie Your Shoes
• Futility Closet: "an idler's miscellany of compendious amusement"