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. [
   http://erika.sssup.it/

EE Times: Open source code smooths RTOS design
   Benefits of open source compared to other RTOS licensing models.
   http://www.techweb.com/se/directlink.cgi?EET19990726S0076

Katix
   Realtime OS (Katix RTOS) and Katix Embedded Linux (Black Cat Linux). [Open source]
   http://www.katix.org/

Linux Journal: Open Source Software for Real-Time Solutions
   Compares 2 open source OSs for embedded uses: RTLinux, eCOS by Cygnus.
   http://www2.linuxjournal.com/lj-issues/issue66/3576.html

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: ne
   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/

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/

Roadrunner
   One-user, general purpose OS built on a protected memory, POSIX threads RT kernel; full kernel, device drivers, file systems, and network protocols runs in about 200 K RAM; now runs on x86 CPUs but designed for easy portability. [GPL]
   http://www.cornfed.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. Der
   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/

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/