Submicrokernel

dr'ex
Exokernel written in C/asm for the x86 architecture. Released under GPL.
http://drex.sourceforge.net/

dr'ex: SourceForge
Information, news, forum, CVS, downloads. [Open Source, GPL]
http://sourceforge.net/projects/drex/

Elysium
Main idea: enforce no abstractions, rather, have them as options, to all levels of system: hardware, kernel and file services, ways users interact with system; based on exo principles. Descriptions, news. [Open Source]
http://elysium.sourceforge.net/

Exokernel
Growing article, with links to many related topics. Wikipedia.
http://en.wikipedia.org/wiki/Exokernel

Exokernel: An Operating System Architecture for Application-Level Resource Management
Traditional OSs limit application performance, flexibility, functionality by fixing interfaces and implementations of OS abstractions such as interprocess communication and virtual memory. Exokernel address this via application-level management of physical resources. [ResearchIndex]
http://citeseer.ist.psu.edu/engler95exokernel.html

Miranda
Planned features: Exokernel architecture, object-oriented, POSIX compliant, intuitive GUI, and best technologies: journaled main filesystems, LibOS modular library. Development documentation. [Open Source, LGPL]
http://miranda-os.sourceforge.net/

Miranda: SourceForge
Downloads, announcements, and a forum. Programming languages: Assembly, C, and C++.
http://sourceforge.net/projects/miranda-os/

miray Software
Makes µnOS: 72k microkernel on 8k nanokernel client/server architecture based on OOP framework, symmetric multithreading, multitasking, priority based scheduling, fully interruptible, separated address spaces, and full memory protection. Free download.
http://www.miray.de/home/

MIT Exokernel OS
Puts applications in control, runs 10x or more faster. Exopc and XOK versions run on x86 PCs. ExOS library gives user-level extensible implementation of Unix OS, so most applications compile and run with no change. Download. [Open Source, MIT]
http://www.pdos.lcs.mit.edu/exo.html

Nanokernel
Growing article, with links to many related topics. Wikipedia.
http://en.wikipedia.org/wiki/Nanokernel

Unununium Operating Engine
Claimed as radical new approach to OSs: no-kernel, self-modifying cells (like objects); single address space, useful where memory and CPU power is low, and maybe for AI. Written in self-modifying, modular assembly language. [Open Source, BSD]
http://uuu.sourceforge.net/