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.
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.
Haiku (formerly known as OpenBeOS) is an operating system designed from the ground up for desktop computing. Inspired by the BeOS, it aims to provide users of all levels with a personal computing experience that is simple yet powerful, and free of any unnecessary complexities. It aims for source and binary compatibility with BeOS R5.
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.
Contiki is an open source, highly portable, networked, multi-tasking operating system for the Internet of Things. Contiki includes a multitasking kernel, a TCP/IP stack and a set of application programs, and a low-power radio communication stack. It is written in C and designed to be very small: it runs comfortably in a few kilobytes of RAM.
UNIXem is a small and simple library that provides emulation of several popular Unix API functions on the Win32 platform. Its primary purpose is to assist Win32 programmers who are porting to Unix or are writing multi-platform code. API functions include opendir/readdir/closedir, glob/globfree, readv/writev, pathconf/realpath, mmap/munmap/msync, dlopen/dlclose/dlsym/dlerror, gettimeofday, and getpagesize.
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.
Warewulf is an operating system management toolkit designed to facilitate large scale deployments of homogeneous and heterogeneous systems on physical, virtual and cloud based infrastructures. Originally, the Warewulf project pioneered the concept of stateless computing in HPC, setting the standard for large-scale cluster provisioning. It provided two functions, provisioning and monitoring but the two functions did not communicate within Warewulf itself, nor was it possible to hook other functions directly into Warewulf itself. Today, Warewulf is more than just a basic provisioning and monitoring solution as it now implements an abstract, object-oriented data store and a modular interface that facilitates a highly extensible, customizable feature set. Current and planned modules include monitoring (operating system, services, filesystems, etc.), provisioning, power management, user management, configuration management, event/trigger handling and notification, scheduler integration, cloud services (both local and remote), etc.