AVRClips is a project mainly written in C, it's free.
Lots of useful libraries for programming AVRs
AVR Clips Kenneth Finnegan 2010
This is a collection of useful code snippets meant for Atmel's AVR 8-bit microcontrollers. Usage for each will be documented online at: https://sites.google.com/site/avrclips/
Most of these code clips are targeted at the ATMEGA family, mainly the mega168 and mega328p