Most of us learned that Arduino is open boards, software and support on this site
For Avr micro-controler, software and IDE until 1.05 are a Wiring Project creation (Hernado Barragan).
Have a look on the Arduino IDE avr files you will find many names like wiring.c, wiring_analog.c, wiring_digital.c, wiring_pulse.c, wiring_shift.c, Winterrupts.c and so on...
ARM:
For ARM (cortex M3 (DUE) and cortex M0 (Zero), software and IDE> 1.5 are only an Arduino creation (eventually with Atmel help for software).
For ARM the only source of support are Arduino and Atmel website.
Notice that Atmel has soon Xplained boards based on SAMD20 and SAMD21.
Notice that Arduino LCC Zero board schematic is not signed by Arduino LLC but by Atmel and board are manufactured in Europe (by Atmel or under Atmel responsabily).
AVR :
For avr Arduino website is the main source of support, but it is not alone.
Wiring site www.wiring.org.co is not very active but it is not dead.
Notice that Avr Wiring code is often more recent that Arduino avr code.