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++.
WinAVR – OSS for Atmel AVR RISC
WinAVR contains all the tools for developing on the AVR. This includes avr-gcc (compiler), avrdude (programmer), avr-gdb (debugger), and more!
You can try WinAVR out with Portable WinAVR, a version that doesn’t require an install.