Nuttx is a real-time embedded operating system (RTOS). It has a small footprint that is usable in micro-controller environments. It is fully scalable from tiny (8-bit) to moderate embedded (32-bit) systems. It also aims to be fully compliant to standards, to be fully real time, and to be totally open.
Mnix is a free, simple, and fast i686 GNU/Linux distribution, aimed at experienced users. It is a hybrid distribution; both precompiled packages and sources are supplied. The main focus is keep it simple and to be as Unix-like as possible, using "Free Software" only. Mnix is installed as a basic system with console-only tools, which forms the base which lets the user build a customized distribution (and even remaster it). Its main features are: Free Software only, BSD-style init scripts, only one shell (bash), a simple package manager (mtpkg), a ports-like repository structure called "mars" (the Mtpkg Applications Repository System), a simple filesystem hierarchy which adheres to the Unix philosophy, kernel-libre-only sources, a set of libraries and compilers for the most-used programming languages, and a complete set of shell packages (installable from the ISO) to set up in 30 minutes (more or less) a fully working console-based system. Mnix GNU/Linux is suitable for the somewhat-experienced user who prefers console admin tools to tweak the system, who prefers to compile packages with custom settings, and who wants to customize the kernel for his own system.
ClearOS is an integrated network server gateway solution for small and distributed organizations. The software provides all the necessary server tools to run an organization including email, anti-virus, anti-spam, file sharing, groupware, VPN, firewall, intrusion detection/prevention, content filtering, bandwidth management, multi-WAN, and more. You can think of it as a next generation small business server. Through the intuitive Web-based management console, an administrator can configure the server software along with integrated cloud-based services.
amforth is an extendible command interpreter for the Atmel AVR ATmega microcontroller family. It has a turnkey feature for embedded use as well. It does not depend on a host application. The command language is an almost compatible ANS94 forth with extensions. It needs less than 8KB code memory for the base system. It is written in assembly language and forth itself.
MikeOS is an operating system for x86 PCs, written in assembly language. It is a learning tool to show how simple OSes work, with well-commented code and extensive documentation. It features a text-mode dialog-driven user interface, a file manager and machine code monitor, and over 60 system calls for use by third-party programs.
Briefly, FreeBSD is a UNIX operating system based on U.C. Berkeley's 4.4BSD-lite release for the i386 platform (and recently the alpha platform). It is also based indirectly on William Jolitz's port of U.C. Berkeley's Net/2 to the i386, known as 386BSD, though very little of the 386BSD code remains. A fuller description of what FreeBSD is and how it can work for you may be found on the FreeBSD home page.