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/
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/
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/
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/
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/
NicheTask - A multitasking embedded operating system with a proven track record, expressly designed for embedded devices with limited resources. - http://www.freertos.com
Katix - Realtime OS (Katix RTOS) and Katix Embedded Linux (Black Cat Linux). [Open source] - http://www.katix.org/
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/
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/
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/
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/