PikeOS
A real-time operating system and hypervisor in one kernel, built so safety-critical and ordinary software can share a processor without interfering.
PikeOS comes from the German company SYSGO, acquired by Thales in 2012. The kernel is a separation kernel in the L4 microkernel tradition: it does very little itself, but it isolates partitions in both memory and time. Each partition runs its own "personality" — an ARINC 653 API, POSIX, a Linux guest or another RTOS.
The point is certification. Proving that an entire Linux install is bug-free is impossible; proving that it cannot touch the partition next door is tractable. PikeOS holds certifications including DO-178C for aviation, EN 50128 for rail and ISO 26262 for automotive, and runs in avionics, trains and defence systems.