The Angstrom Distribution

Angstrom was started by a small group of people who worked on the OpenEmbedded, OpenZaurus and OpenSimpad projects to unify their effort to make a stable and userfriendly distribution for embedded devices like handhelds, set top boxes and network-attached storage devices and more.

ttylinux

The goal this project is to make one of the smallest, up-to-date Linux systems, yet keep it similar to a larger distribution.

Planet Open Source

DSPnano RTOS and Unision RTOS are Linux or POSIX compatible real-time operating systems specifically geared for DSP applications (i.e., applications that process data on a continuous basis in real-time as it flows to and from external devices.) The DSPnano RTOS is tiny and runs on many 16-bit microcontrollers, DSPs (digital signal processors) and DSCs (digital signal controllers). Unision DSP RTOS is more general; it is suitable for embedded processors that have 32-bit buses.

TinyOS

TinyOS is a free and open source component-based operating system and platform for wireless sensor networks. TinyOS system, libraries, and applications are written in nesC, a new language for programming structured component-based applications. The nesC language is primarily intended for embedded systems such as sensor networks.

BeRTOS

BeRTOS is a real time open source operating system supplied with drivers and libraries designed for the rapid development of embedded software.

Platform: unknown; License: GNU General Public License, version 2

Reference: https://www.bertos.org/

Contiki

Contiki is an open source, highly portable, multi-tasking operating system for memory-efficient networked embedded systems and wireless sensor networks. Contiki has been used is a variety of projects, such as road tunnel fire monitoring, intrusion detection, water monitoring in the Baltic Sea, and in surveillance networks. Contiki is designed for microcontrollers with small amounts of memory. A typical Contiki configuration is 2 kilobytes of RAM and 40 kilobytes of ROM.

Platform: unknown; License: BSD License

Reference: https://www.sics.se/contiki/about-contiki.html

XMK (eXtreme Minimum Kernel)

eXtreme Minimal Kernel is a preemptive multithreaded real-time operating system for microcontrollers. XMK's primary design goal is to be small, extremely small, without sacrificing performance or functionality.  XMK's minimal footprint makes it ideal for running on 8bit microcontrollers, while its feature content makes it a excellent choice for 16bit and 32bit microcontrollers.

Platform: unknown; License: Shift-Right's Open Repository licensing

Reference: https://www.shift-right.com/xmk/

MINIX 3

MINIX 3 is a new open-source operating system designed to be highly reliable, flexible, and secure. It is loosely based somewhat on previous versions of MINIX, but is fundamentally different in many key ways. MINIX 1 and 2 were intended as teaching tools; MINIX 3 adds the new goal of being usable as a serious system on resource-limited and embedded computers and for applications requiring high reliability.

Platform: unknown; License: BSD License

Reference: https://www.minix3.org/

Inferno

InfernoR is a compact operating system designed for building distributed and networked systems on a wide variety of devices and platforms. With many advanced and unique features, Inferno puts an unrivalled set of tools into your hands. You can fetch it now as Free Software, on similar terms to Linux or xBSD.

Platform: unknown; License: GPL or LGPL / Free Software Scheme or Commercial Developer Licence

Reference: https://www.vitanuova.com/inferno/index.html

FreeRTOS.org

FreeRTOSTM includes official ports to 26 architectures and receives more than 77,500 downloads a year. It is a popular, portable, open source, royalty free, mini Real Time Kernel - a free to download and free to deploy RTOS that can be used in commercial applications without any requirement to expose your proprietary source code.

Platform: unknown; License: Modified GPL (Open Source) Licensing

Reference: https://www.freertos.org/

eCos - embedded Configurable operating system

eCos is an open source, royalty-free, real-time operating system intended for embedded applications. The highly configurable nature of eCos allows the operating system to be customised to precise application requirements, delivering the best possible run-time performance and an optimised hardware resource footprint.

Platform: unknown; License: Modified GPL (Open Source) Licensing

Reference: https://ecos.sourceware.org/

uClinux

The original uClinux was a derivative of Linux 2.0 kernel intended for microcontrollers without Memory Management Units (MMUs). However, the Linux/Microcontroller Project has grown both in brand recognition and coverage of processor architectures. Today's uClinux as an operating system includes Linux kernel releases for 2.0 2.4 and 2.6 as well as a collection of user applications, libraries and tool chains.

Platform: unknown; License: GNU General Public License (GPL)

Reference: https://www.uclinux.org/description/