List of free programming languages and tools.

AVR microcontrollers

BASCOM-AVR - BASIC windows compiler. Most of the examples are written in BASCOM. Free version compiles 4kb of code.
WinAVR - free and fully functional C compiler for AVR. Linux version available.
Atmel AVR Studio 4 - integrated development tool. It contains editor, debugger and simulator. It can compile programs using avr-gcc. Note that current version of AVR Studio has many bugs, but it is still a very good tool.
AVaRICE Linux program which interfaces the GNU Debugger GDB

AVR C libraries

AVRlib - is a library of easy-to-use C functions for a variety of common and uncommon tasks using AVR processors.
Peter Fleury software examples. Good I2C master library.
General peripheral drivers by Chris Efstathiou. High level functions for various devices, e.g. DS18B20/1-wire.
Martin Thomas AVR-Projects, among others a good example for 1-wire DS18B20 thermometer.