ATmega-Skeleton is a project mainly written in C and SHELL, it's free.
Skeleton libraries for AVR ATmega boards such as the arduino
ATMega Skeleton: Skeleton libraries for AVR ATmega boards such as the arduino.
To contribute, get started with the following:
You will need avr-gcc toolchain, make and some other packages. On a Debian clone following will install the build-essentials.
$ sudo apt-get install make gcc-avr binutils-avr avr-libc
Optionally you can install "avrdude" an AVR controller programming tool:
$ sudo apt-get install avrdude libusb-dev
The following might help: