Xinu
Name is the reverse of Unix. Small, elegant, multitasking OS with concurrent processing, message passing, ports, semaphores, memory management, buffer pools, uniform device I/O, shell, Tcl, TCP/IP. Begun as means to teach OS design concepts, now used by many education institutions; some embedded system makers have ported it to their hardware: Intel 80960, 8096; Motorola 680x0.