Effort to create a POSIX-like operating system atop the rtmk microkernel. [Open Source, GPL]

Small Unix-like microkernel OS, process- and file-oriented, most of the system is built around the scheduler; designed to use minimum system resources to provide network services and utilities; written in C; source browsing and downloads available. [Open Source, GPL]

A POSIX microkernel with roots of the Mach and FreeBSD projects. [Open source, BSD-like]