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]
ERIKA Enterprise
A commercial realtime system with minimal footprint that supports advanced scheduling mechanisms for stack size reduction on multiprocessor-on-a-chip systems.
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. Site has fine 'Getting Started' section: readme, release notes, installation, hardware, compatibility, troubles Q&A, contacts.
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]
Sphere SP
A modular RTOS suitable for embedded systems, scalable from tiny stand-alone microkernel up to full client/server OS, supports different processors.
Spring Project
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.