Microkernel

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. [Open Source, GPL]
http://erika.sssup.it/

ERIKA Enterprise
Embedded RTOS for single and multi-core uses, small size, supports advanced scheduling mechanisms for stack size reduction on multiprocessor chip systems. Commercial. Evidence S.r.l.
http://www.evidence.eu.com/content/view/24/45/

Phoenix-RTOS
Goals: develop free, portable, small, well-designed RTOS for embedded hardware platforms: SBC (Single Board Computers), SOM (System on Module), SoC (System on Chip). Free.
http://www.phoenix-rtos.org/

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

PikeOS
Uses virtualization to protect software investment long-term; implements multiple software partitions on one CPU, strictly separated, each partition can run full OS such as Linux, POSIX, OSEK, or programs. Sysgo AG; Mainz, Germany.
http://www.sysgo.com/products/pikeos-technology/

Prex
Portable, small, realtime microkernel 32-bit OS for embedded systems, coded in C, POSIX emulation layer. Open source, BSD.
http://prex.sourceforge.net/

Real-Time Mach NTT
Another version of Real-Time Mach, by NTT Laboratories. Contains RT-Mach kernel, FreeBSD 2.2.8 server, real-time shell, RT-Java, Hurd loader. Getting Started section: readme, release notes, installation, hardware, compatibility, troubles questions and answers, and contacts.
http://info.isl.ntt.co.jp/rtmach/

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. [Open Source, GPL]
http://rtmk.sourceforge.net/

Sphere SP
A modular RTOS suitable for embedded systems, scalable from tiny stand-alone microkernel up to full client/server OS, supports different processors.
http://www.miray.de/products/spheresp.html

Spring Operating System
Growing article, with links to many related topics. Wikipedia.
http://en.wikipedia.org/wiki/Spring_(operating_system)

An Overview of the Spring System
Description of realtime microkernel and integrated environment, designed and implemented to support/provide predictability, guarantees (on-line dynamic, atomic), end-to-end scheduling, resource reservations; supports call/task admission model; and multiprocessors. Proceedings of Compcon. <small>(February, 1994)</small>
http://research.sun.com/features/tenyears/volcd/papers/mitchell.htm