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 RTOS  
   Free, tiny.
   http://www.funet.fi/~kate/katix.html

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/

MenuetOS  
   Graphical realtime OS written in all 32-bit assembly, for x86 i386+, fits on one floppy, protected memory, 44 kHz sound soon, GUI, DOS loader download; advancing well. [Open Source, GPL]
   http://www.MenuetOS.org/

Roadrunner: Cornfed Systems, Inc.  
   Makes Roadrunner/pk open source (GPL), 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
   http://www.cornfed.com/

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

RTX: embeddable Real Time Executive  
   Ideal for tiny, low power devices; coded in C; permits embedding OS in any application for multi-threading. This works if host is Linux, Windows 95/98/Me/NT/2000, or an embedded project with no OS. If applications run in Linux or Windows, RTX provides no
   http://world.std.com/~mikep/rtx-page.html

CarbonKernel  
   Open Source 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/

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/

FreeRTOS  
   Portable, mini realtime scheduler (RTOS kernel); how to make full embedded solutions on Windows host with quality open source tools, low cost hardware; supports Atmel(Mega)AVR, RDC8822 (AMD AM186 clone), PC with DOS. [Open Source, GPL]
   http://www.freertos.org/