GCC

GCC screenshots

GCC was originally written as the compiler for the GNU operating system

Alternative downloads

WinAVR
WinAVR includes avr-gcc , avrdude (programmer), avr-gdb (debugger), and more!
FREE
AVR Studio
Powerful tool designed for programming microcontrollers.
5.1.208
FREE
GCC - Open Source Compiler for MSP430 Microcontrollers
Debugger and open source C/C++ compiler for MSP430 microcontrollers.
4.1.0.0
FREE