Exolu is a separation of the Linux kernel into two parts: an exokernel and a library. The exokernel consists only of the implementations for basic hardware resource access and drivers, but no abstractions. The library implements the abstractions and interfaces that are used by programs to interact with the kernel.
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.
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.
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.
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.
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.
HelenOS is a microkernel-based multiserver operating system designed from scratch. By decomposing the operating system functionality into tens of isolated but intensively communicating userspace servers, it provides a computing environment that has several virtues such as flexibility, increased robustness, well defined explicit interfaces, and smaller complexity of individual components. HelenOS does not aim to be another clone of Unix or some other legacy system and is not POSIX-compliant (even though it may seem POSIX-similar at times). Instead, the goal has been to design it according to what is the most elegant and right thing to do. What makes HelenOS unique among the other multiserver operating systems is its multiplatform and multiprocessor microkernel. It will run on seven different processor architectures ranging from a 32-bit uniprocessor little-endian ARMv4 to a 64-bit multicore big-endian UltraSPARC T1.
Buenos is a small SMP operating system skeleton that can be used as a base for operating systems project courses. The system is intended to be used as OS project base code which students can improve. Extensive documentation with suggestions for assignments or exercises is included with the system. It runs in a machine simulator called YAMS, which is its sister project.