EE Times: Open source code smooths RTOS design
Benefits of open source compared to other RTOS licensing models.

Katix RTOS
Free, tiny.

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.

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]

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 portability.

Linux Journal: Open Source Software for Real-Time Solutions
Compares 2 open source OSs for embedded uses: RTLinux, eCOS by Cygnus.

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 hard-real time as direct interrupt mapping to threads is prevented.

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]

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]

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]