Brian J. Swetland's _(OpenBLT | http://www.frotz.net/openblt) cleanroom implementation of a _(microkernel). Conventional implementation of processes and threads, but the codebase was written from scratch and works to some extent. A nice project to join if you want to help build an *(OS) that already went beyond the bootloader stage. The system is updated on a regular basis, seems to have gathered a few more developers and development seems nicely organised.