HelenOS
A microkernel-based multiserver operating system written from scratch.
...HelenOS is therefore flexible, modular, extensible, fault tolerant and easy to understand.
HelenOS does not aim to be a clone of any existing operating system and trades compatibility with legacy APIs for cleaner design. Most of HelenOS components have been made to order specifically for HelenOS so that its essential parts can stay free of adaptation layers, glue code, franken-components and the maintenance burden incurred by them.