Open Source

Apostle
Free microkernel operating system for i386 architecture, built on Gemini Nucleus, coded in C. Seeks to provide well-designed alternate to extant OSs; goal: create an OS that works because it is appropriate and effective for its users. [open source, GPL]
http://apostle.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. [GPL]
http://erika.sssup.it/

Gemini Nucleus
Open source project to make a production quality micro kernel based on the latest findings in OS research.
http://gemini.sourceforge.net/

JTMOS
Operating system aimed at OS developers and users who are very advanced at using computers generally; microkernel, multitasking, multithreaded, some garbage collection. [Open Source]
http://www.fortunecity.com/tattooine/obiwan/32/

LinksOS
To be fast, secure, straightforward, fill gap between OS X, Linux, Windows, built on microkernel, coded in Assembly, C/C++; for desktops, workstations, small servers. Description, mail list, OS design documents, CVS, downloads. [Open Source, BSD]
http://sourceforge.net/projects/linksos/

Mika OS
Low-level coded, AI oriented microkernel OS, based on neural nets. Goal: provide a hight flexible OS architecture based on neural networks and sensor equipped devices. Mach4/Flux-based. [Open Source, GPL]
http://mikaos.ifrance.com/mikaos/

Moo32
Early development microkernel.
http://www.geocities.com/SiliconValley/Vista/7477/os.html

NewOS
Loosely based on concepts in BeOS, FreeBSD, NT, Solaris, other modern OSs; goals: be as modern and complete as possible, emphasize platform portability; runs on x86 1, 2/4 SMP CPUs, Sega Dreamcast Hitachi SH-4. Kernel used in OpenBeOS. [Open Source, BSD]
http://newos.org/

OS/C
Small POSIX microkernel OS, process- and file-oriented, most of the system is built around the scheduler; designed to use minimum system resources to provide network services and utilities; written in C; source browsing and downloads available. [Open Source, GPL]
http://blake.prohosting.com/osc1/

Programmers Operating SYStem: POSYS
To be simple 32-bit multitasking microkernel using all security features of x86 processors, able to load and execute modules for: MS Win32 (32-bit PE, EXE; dynamic link libraries, DLLs), Linux (ELF, a.out). [Open Source, GPL]
http://posysos.sourceforge.net/

ReactOS
Opensource effort to develop a quality operating system compatible with Windows NT applications and drivers.
http://www.reactos.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/

sMultiTA
World's smallest, but full functioning, software extension for MultiTAsking. Very small 200 lines, coded in C, simple, no interrupts or blocking, for hard realtime use; portable, easy to reconfigure to other systems. [Open Source, GPL]
http://www.dobit.com/emblin/smultita.htm

The Spoon Microkernel
Hobby and education OS, for Intel compatible CPUs. Descriptions, documents, screenshots, tips, links. [Open Source, BSD]
http://www.smk.co.za/