Electronics Reference Materia

• IAD WIki - All essential material for Physical Computing
•  - Great video series on the basics of electricity. 
• Elektronik Compendium – Excellent explanations of electronics (Deutsch)
• All about circuits -  In-depth explanations of electricity, from basic to advanced topics
• Tom Igoe – Many good explanations of basics
• Find Chips – The place to find datasheets for components 

Websites

• hackaday - A news site for interesting "hacking" projects. A source of inspiration and new developments
• ITP – The very good documentation from the Interactive Telecommunications Program (ITP) at NYU
• makezine.com – A magazine for hobbyist, with many Arduino projects
• JeeLabs – Several interesting projects from Jean-Claude Wippler.

Hardware

• Arduino Boards – An overview of all Arduino boards 
• raspberrypi - A small, cheap, pc computer suitable for hardware prototyping 
• Sparkfun –  Electronic prototyping designer/manufacturer with many tutorials. 
• Adafruit – Electronic prototyping designer/manufacturer with many tutorials.