Links "a la carte"

- Interfacing many things with Ardunio Great tutorials at tronixstuff
- Controlling and Displaying Arduino on Android: Complete demo code in Processing and explanation to communicate over Serial between an Arduino (or any microcontroller) with Bluetooth Original in Spanish or translated in English.
AVR and ATtiny resources
- Setup and programming an ATtiny10 with an Arduino
- AVR GCC programming tutorial, great to get started
- An Introduction to Practical Electronics, Microcontrollers and Software Design. Thousand pages guide about electronics and AVR programming. Very good resource for getting started or brushing-up. 

Projects I particularly noticed
Centeye vision chips, to compute optical flow with an Arduino.
Open Source Laser Projection Keyboard a virtual keyboard is projected on a table and image recognition "see" where the fingers are applied. This company makes a more refined product, but hardware and software are closed.
- Open Energy Monitor

In Japan
Tokyo HackerSpace guide to Akihabara, this is the definitive guide to the electronic mecca in Tokyo. Enjoy the tour.

Electronic Shops (Some original shops on the net)
Little Bird Electronics Great shop in Australia with stuff from bigger shops, they sell the Freetronics Arduino
- SeeedStudio, some original sensors and projects. In addition their PCB service is cheap and reliable.
- Akizuki Denshi, a big reseller in Japan with a small shop in Akihabara but a very large choice of electronic components. A good addition to Digikey.

Breadboard Arduino (Some inspiration to build the Petitduino)
RBBB The Really Bare Bone Board. Similar with the Petitduino with a nice trick; it is possible to remove some parts and make it even smaller
Ardweeny  A very small and cheap board. Just the minimum features.
Arduino on Breadboard  A good guide to see how all the components need to be connected
Boarduino  The name says it all, comes in different versions with USB or external supply