chaOS, like OpenBLT is a cleanroom implementation of an OS based on a microkernel (codenamed Storm), announced over the summer; there is both code and documentation available on the site, as well as a downloadable and bootable demo floppy that will let you try your hand at it in no time. The project is expanding very quickly and the core team motivation doesn't seem to be dying, nor are their programming abilities. Definitely a project to follow.