Hardware
arduino.cc
particle.io
adafruit.com
sparkfun.com
shrimping.it
littlebits.cc
raspberrypi.org
wink.com
smartthings.com
makeymakey.com
Tutorials
arduino.cc/en/Tutorial/
learn.adafruit.com
learn.sparkfun.com/tutorials
hackster.io
bento.io
All About Electronic Circuits for Kids (thank you Emma)
Prototyping Software
123d.circuits.io
fritzing.org
kicad-pcb.org
ifttt.com
zapier.com
netlabtoolkit.org
Inspiration
creativeapplications.net
hackaday.com
iotlist.co
postscapes.com
exciting.io/printer
HomeMirror
CatBot
Arduino + HTML5 Etch-a-Sketch
Seinfeld Door
Javascript Interfaces
johnny-five.io
SparkJS
cylonjs.com
Misc
Processing + Arduino
Arduino Cheatsheet
techprep.fb.com
wolfram.com/programming-lab