Finnix is a small, self-contained, bootable Linux CD distribution ("LiveCD") for system administrators, based on Debian testing. You can use it to mount and manipulate hard drives and partitions, monitor networks, rebuild boot records, install other operating systems, and much more.
Linux is a clone of the Unix kernel, written from scratch by Linus Torvalds with assistance from a loosely-knit team of hackers across the Net. It aims towards POSIX and Single UNIX Specification compliance. It has all the features you would expect in a modern fully-fledged Unix kernel, including true multitasking, virtual memory, shared libraries, demand loading, shared copy-on-write executables, proper memory management, and TCP/IP networking.
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.
musl is a new implementation of the standard library for Linux-based systems. It is lightweight, fast, simple, free, and strives to be correct in the sense of standards-conformance and safety. It includes a wrapper for building programs against musl in place of the system standard library (e.g. glibc), making it possible to immediately evaluate the library and build compact statically linked binaries with it.
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.
Owl (Openwall GNU/*/Linux) is a small security-enhanced Linux distribution for servers. Owl also makes a good base system for customized virtual machine images and embedded systems, and Owl live CDs with remote SSH access are good for recovering or installing systems (whether with Owl or not). A single Owl CD includes the full live system, installable packages, the installer program, as well as full source code and the build environment capable of rebuilding the entire system from source. Owl supports multiple architectures (x86, x86-64, SPARC, and Alpha) and offers some compatibility for packages developed for other Linux distributions. The primary approaches to security are proactive source code review, privilege reduction, privilege separation, careful selection of third-party software, safe defaults, and "hardening" to reduce the likelihood of successful exploitation of security flaws.