Real-Time Operating System for Multiprocessor Systems.
Version: 4.9.1RTEMS (Real-Time Executive for Multiprocessor Systems) is a commercial grade real-time operating system designed for deeply embedded systems. The project is a free (as in beer and speech) open-source solution that supports multi-processor systems.
Operating System: Linux
RTEMS is designed to support applications with the most stringent real-time requirements while being compatable with open standards such as POSIX. It's been ported to over ten processor architectures including ARM, PowerPC, i386, M68K, Coldfire, MIPS, SPARC, SH, H8, and C3x/C4x DSPs.
Using the GNU tools for cross development, you can use any number of systems as development hosts including both Unix and Windows platforms.