Microkernel

BugOS
New microkernel operating system for x86 computers, many features: network and Internet modules, file system (FAT); kernel handles memory, tasks, micro kernels, and some base devices: keyboard, video, hdd, and ramdrive.
http://bugos.nop.hu/

Cosy
Goal: scalability of highly parallel multicomputer systems. Based on small microkernel that does process management, interprocess communication; all other services are processes out of kernel. Processes and address spaces orthogonal, so Cosy process is like thread in other OSs. Hardware dependencies concentrated in areas of kernel for portability.
http://wwwcs.uni-paderborn.de/fachbereich/AG/heiss/cosy/

K42
Growing article, with links to many related topics. Wikipedia.
http://en.wikipedia.org/wiki/K42

K42
High performance, general-purpose research OS kernel for cache-coherent multiprocessors, for next generation servers ranging from small-scale (to grow ubiquitous), to very large-scale non-symmetric (growing important in commercial and technical environments. IBM Research. Open source.
http://domino.research.ibm.com/comm/research_projects.nsf/pages/k42.index.html

K42 Project Wiki
Large, growing resource of text information.
http://k42.ozlabs.org/Wiki/

The MicroEmpix Fan Site
One user microkernel (nearer an exokernel), very small, version of Empix Unix-like OS developed at the Computing Systems Laboratory, National Technical University of Athens.
http://www.softlab.ece.ntua.gr/~kalabkas/mempix/indexold.html

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

The Open Group: Advanced Research
Page listing several research projects: microkernel OSs (MK7, MK++, AD3) and other modules (CONVERSANT, CORDS, GIPC, SHAWS).
http://www.osf.org/RI/

Sartoris Microkernel Tutorial
Explains about making a policy-free microkernel; multipart series.
http://sartoris-microkernel.blogspot.com/2006/06/0.html

ShagOS
Portable object-oriented microkernel OS, dynamically loaded device drivers, fully redesigned and rewritten many times in C++, runs on VAX, x86. Ongoing experiment in using O-O paradigm as framework for full OS, with distributed computing as main aspect in most design decisions.
http://www.csh.rit.edu/~shaggy/shagos.html

SUMO
SUpport for Multimedia in Operating systems, Lancaster University: microkernel OS with facilities to support distributed realtime and multimedia applications and ODP-based multimedia distributed application platforms.
http://www.comp.lancs.ac.uk/computing/research/sumo/

Topsy
Teachable Operating System: tiny multithreaded messaging microkernel, in ANSI C; protected threads, memory managed, and thread/process control. From undergraduate course on concurrency, device programming, OS concepts. Descriptions, documents, theses, downloads, contacts, links. [Open Source, GPL]
http://www.tik.ee.ethz.ch/~topsy/