OpenBLTBrian J. Swetland's 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.
This page is linked from: chaOS