The playground is a publicly-editable wiki about Arduino.
Manuals and Curriculum
Board Setup and Configuration
Arduino on other Atmel Chips
Interfacing With Hardware
Interfacing with Software
User Code Library
Suggestions & Bugs
Sources for Electronic Parts
Related Hardware and Initiatives
Arduino People/Groups & Sites
There are currently two open-source print documentation projects in production that cover various aspects of the Arduino microcontroller.
Arduino Booklet written by Massimo Banzi in an early release can be found here or here (pdf).
Arduino Programming Notebook written by Brian Evans in its first edition can be