RTEMS (Real-Time Executive for Multiprocessor Systems) is a complete RTOS, including kernel and a wide set of services
such at TCP-IP stack (FreeBSD), Telnet server, FTP server, Filesystem, CLI, PSOIX layer etc. RTEMS has support for
multiple architectures such as intel, powerpc, arm, sparc, m68k etc.. The set of BSPs available is limited but most
architecture runs under simulators. RTEMS community seems very helpful and active with multiple on-going projects.
The GNU compiler for RTEMS is available for both Linux [R1] and Windows (Cygwin [R2] or MinGW [R3]) development environment. Various Linux distributions are supported with their native installation packages. RTEMS documentation is very riched including tones of WIKI pages around. RTEMS has some nice features such as the built-in Shell but is lacking of a configuration GUI compare to eCos RTOS for instance.
RTEMS based applications can be loaded with usual bootloader designed for embedded platform (i.e. redboot , u-boot , barebox  etc.).
[R1] - Toolchain for Linux
[R2] - Toolchain for Cygwin
[R3] - Toolchain for MiniGW
[R4] - HOWTO: RTEMS / MiniGW
[R5] - Eclipse plug-in
[R6] - Getting started
[R7] - RTEMS C User's Guide
[R8] - RTEMS Shell User's Guide
[R9] - RTEMS Filesystem User's Guide
[R10] - RTEMS Networking User's Guide
[R11] - Supported architecture