NewOsAn open source OS with an emphasis on design and portability; it is largely implemented in C/C++, with a small amount of assembly.
Currently, the system is mostly a kernel with a minimal amount of user space libraries and applications. Thus far, most of the work has been put into the kernel and other underlying support. As a result the system isn't that interesting from an end-user point of view (no gui, simple commands on a command line), but that can change given a moderate amount of work by the maintainers or any volunteers.
Used in OpenBeOS.