WinAVR (pronounced “whenever”) is a suite of executable, open source software development tools for the Atmel AVR series of RISC microprocessors hosted on the Windows platform. It includes the GNU GCC compiler for C and C++.
Check out the WinAVRTM project on Sourceforge at http://sourceforge.net/projects/winavr/ for developer information. Use the navigation menu on the left to find your way around this site.
WinAVRTM contains all the tools for developing on the AVR. This includes avr-gcc (compiler), avrdude (programmer), avr-gdb (debugger), and more! WinAVR is used all over the world from hobbyists sitting in their damp basements, to schools, to commercial projects.
WinAVRTM is comprised of many open source projects. If you feel adventurous, volunteers are always welcome to help with fixing bugs, adding features, porting, writing documentation and many other tasks on a variety of projects.
http://winavr.sourceforge.net/
You can use Eclipse as an IDE, see Eclipse IDE for Embedded AVR Software Development