Open Source

CarbonKernel
RTOS simulator based on event-driven simulation methods with concurrent target debugging abilities, mimics OS behavior, to implement and test embedded software on workstations. Descriptions, downloads, documents. [Savannah, Open Source, GPL]
http://savannah.nongnu.org/projects/carbonkernel/

E.R.I.K.A.
Microkernel RTOSs and set of integrated tools to develop embedded realtime applications, made to support all architectures used in power train controllers in automotive industry. Kernels have 2 main layers: Kernel Layer, Hardware Abstraction Layer, HAL. [Open Source, GPL]
http://erika.sssup.it/

NicheTask
A multitasking embedded operating system with a proven track record, expressly designed for embedded devices with limited resources.
http://www.freertos.com

Nilsen Elektronikk AS
Makes 2 very small, open source realtime OSs for embedded systems, written in C: proc Real-Time Kernel, preemptive, mature; nesos Finite State Machine Operating System (FSMOS), modular, use concurrent programming methods in little time and space. Also: nemon Boot and Debug Monitor; Embedded Web Servers.
http://www.nilsenelektronikk.no/

RET & COM Research
Makes RadiOS, Radiant Operating System; realtime, multitasking, multiuser, for i386+, written in all assembly in NASM. Goal: create OS that combines all advantages of OS/2 and Unix; ftp downloads. [Public Domain]
http://radios.sourceforge.net/

rtmk
Realtime microkernel: preemptive, fully re-entrant, multithreaded, SMP, continuations, full locking primitives; shares many ideas with Mach, developed by CMU in late 1980's, early 1990's, stopped in 1994. [GPL]
http://rtmk.sourceforge.net/

RTMX, Inc.
Makes RTMX O/S: IEEE POSIX realtime extensions to OpenBSD, with vendor specific ports for embedded, dedicated systems, application specific Internet/intranet servers. Many fine features. North Carolina, USA.
http://www.rtmx.com/

S.Ha.R.K.
Dynamic configurable kernel architecture to support hard/soft/non realtime use with interchangeable scheduling algorithms: fully modular in scheduling policies, aperiodic servers, concurrency control protocols; all not modular in most traditional OSs. Derived from HARTIK: HArd Real TIme Kernel. [GPL]
http://shark.sssup.it/

Solar_OS
Also Sol_OS; goals: realtime, coded in 32-bit ASM, fast, very simple, small, cooperative multitasking, programs have full hardware control, modern OOP GUI, suitable for desktop targets. Open source, GPL.
http://www.oby.ro/os/

Linux Journal: Open Source Software for Real-Time Solutions
Compares 2 open source OSs for embedded uses: RTLinux, eCOS by Cygnus. <small>(October, 1999)</small>
http://www.linuxjournal.com/article/3576

EE Times: Open source code smooths RTOS design
Benefits of open source compared to other RTOS licensing models. <small>(July 26, 1999)</small>
http://www.techweb.com/se/directlink.cgi?EET19990726S0076