I have been reading up on microcontrollers quite a bit lately. Following are the links I have found to be informative.

Arduino

• Arduino Electronic Brick Starter Kit and Sensor Shield
• ArduPilot
• OpenSprinkler
• Tiny Circuits
• Use the TP-Link TL-WR703N to send Arduino sensor data
• YABBAS - Yet Another Bare Bones Arduino (on Stripboard)

Raspberry Pi

• 47 Raspberry Pi Projects to Inspire Your Next Build
• Raspberry Pi Car Cam overlaid with OBD data
• AlaMode
• RPi Expansion Boards

Other Microcontrollers

• Freescale mbed
• Pi vs BeagleBone-Black
• Beagleboard on github

Low Power

• Efficiently operate the Arduino GSM shield with a battery pack
• Low Power Library for Arduino
• Power saving techniques for microprocessors
• narcoleptic: a sleep library for Arduino

Stores

• Adafruit
• Gravitech
• Newark
• Sparkfun