Submicrokernel

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

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

Exokernel - Growing article, with links to many related topics. Wikipedia.
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]
citeseer.ist.psu.edu/engler95exokernel.html

KeyKOS Nanokernel Architecture - Thorough architectural analysis of persistent, pure capability, for IBM S/370 mainframes, by Key Logic, Inc; text, diagrams, comparison, references, bibliography, contacts.
www.cis.upenn.edu/~KeyKOS/NanoKernel/NanoKernel.html

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

Miranda: SourceForge - Downloads, announcements, and a forum. Programming languages: Assembly, C, and C++.
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.
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]
www.pdos.lcs.mit.edu/exo.html

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

TUNES Project: No-Kernel - Unique description of operating systems without kernels, links.
cliki.tunes.org/No-Kernel

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]
uuu.sourceforge.net/