Below are some of my 'go-to' sites for information about microcontrollers and related topics.

Arduino

• Arduino on Wikipedia History and plenty of information on Arduino
• arduino.cc Ground zero for all things Arduino
• adafruit.com Lots of helpful tutorials, kits, and components
• sparkfun.com Ditto
• seeedstudio.com "Open hardware facilitator". Often have very good prices on microcontrollers, sensors, and other things useful for mechtronics
• JeeLabs.org "Computing Stuff tied to the Physical World"
• earthshineelectronics.com Arduino Starter Kit Manual Fantastic guide from zero through fairly sophisticated projects
• Arduino tutorials Very detailed and extensive tutorials on Arduino and interfacing it to all sorts of things
• Laboratory for Experimental Computer Science at the Academy of Media Arts Cologne Great examples showing the use of Arduino for measurements, audio, and photography applications
• Tinkerish.com Wiki also Tinkerish.com Wiki Libarduino Libraries that allow developers to use the standard GNU toolchain to program Arduino hardware

Others

• Intro to Physical Computing Tom Igoe’s awesome class on mechatronics
• Dr. Rainer Hessmer’s blog Has some very clearly documented examples relating to robotics and software
• smileymicros.com Joe Pardue’s site that focuses on Atmel Butterfly and Arduino. His blog is good.
• Building Wireless Networks Rob Faludi’s site for the book of the same name, which uses the Arduino. Also has a helpful blog and links to many helpful resources
• AVRfreaks The forum for AVR information
• embedded-lab.com An online teaching laboratory for Microcontrollers and Embedded Systems