MutekH is a portable and free operating system for embedded platforms originally developed at the SoC department of the LIP6 Laboratory in Paris. MutekH is a set of libraries built on top of an exo-kernel designed to support heterogeneous multiprocessor platforms. MutekH is fully configurable to match every application's needs. It is used in several research projects and currently supports x86, arm, mips, and powerpc processors.
MBSL (MicroBlaze Simple Linux) is a set of Makefile scripts to create an image of a Linux-based OS for the Xilinx MicroBlaze (FPGA) soft processor. It provides very simple package management that allows you to customize the resulted image. The purpose is to just type 'make build install' and get the configured image.
The OpenBSD project produces a free, multi-platform 4.4BSD-based UNIX-like operating system. Its goals emphasize portability, standardization, correctness, proactive security, and integrated cryptography. OpenBSD supports binary emulation of most programs from SVR4 (Solaris), FreeBSD, Linux, BSD/OS, SunOS, and HP-UX.
Dragora is a trustworthy GNU/Linux-Libre distribution based on the concept of simplicity with the goal of being a multi-purpose operating system. Dragora respects the freedom of the user with the values of Free Software and provides control to those who use it. It is developed entirely by volunteers and it is published under the terms of the GNU General Public License.
GNU Mach is the microkernel upon which a GNU Hurd system is based. It provides an Inter Process Communication (IPC) mechanism which the Hurd uses to define interfaces for implementing in a distributed multi-server fashion the services a traditional operating system kernel provides. Mach is particularly well-suited for SMP and network cluster techniques. Thread support is provided at the kernel level, and the kernel itself takes advantage of that. Network transparency at the IPC level makes resources of the system available across machine boundaries.