Roaming through the Arduino Source Tree
On my linux system, the bulk of the arduino package gets installed to /usr/share/arduino.
Even if you do not intend to use the Arduino GUI, there are things in here that are worth
taking a look at and knowing about.  Here is a list of things I have found worthwhile:
-  examples -- this has source code for all the Arduino examples.
Although the files have ".ino" extensions, they are sketches.
 -  libraries -- all kinds of things here.  Ethernet, Firmata, SPI, Servo,
SoftwareSerial, Stepper, and more.  mostly ".ino" and ".cpp" files.
 -  hardware/arduino -- a lot of useful things here:
 
 -  bootloaders - source for various bootloaders.
 
 -  cores/arduino - source for HardwareSerial, USB, and more
 
 -  firmwares - source for the Atmega8u2 firmware used on the Uno and others.
 These are LUFA based, and very interesting.
 
 -  variants - pins_arduino.h for leonardo and others.
 
 
 
Documentation
It is always worth looking at:
Feedback? Questions?
Drop me a line!
Tom's Computer Info / tom@mmto.org