Windows
Mac OS X
Linux (on the playground wiki)
Environment: Description of the Arduino development environment (IDE) and how to change the default language.
Libraries: Using and installing Arduino libraries.
Troubleshooting: Advice on what to do if things don't work.
Interfacing with hardware
Collection of information and software resources for interfacing Arduino cards with a wide range of hardware devices at www.arduino.cc
Programming Language reference
Online language reference for Arduino programming, using the free IDE development environment.