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]
miray Software
Makes Sphere SP modular RTOS suitable for embedded systems, scalable from tiny stand-alone microkernel up to full client/server OS, supports different processors.
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]