SunriseOS 
Tiny operating system written in Assembly, via NASM, by ICT student. Meant for only fun and learning. [Open Source]
http://www.geocities.com/sunriseos/KOS
Kid Operating System. Modular (run-time dynamic linking), preemptive multitasking, to have SVR4-like VM. Begun 1998 by young programmers, most of them students. Goal: not obtaining OS, but learning OS internals and functioning of x86 processors. Downloads, links, documentation. [Open Source, GPL]
http://kos.enix.org/
Amateur OS: AMOS
Scott Billingsley's OS: realtime for DSP and amateur radio, one user/application, 16-bit real mode, for i386+, FAT 12/16; no TSRs, library of routines to program into apps, minimal kernel support (minimize interrupts), written in NASM assembly and Sphnix C--.
http://scottie.20m.com/
RCOS
Ron Chernich's OS: computer learning tool animates operation of multi-tasking OS; true OS, runs programs; object-oriented, message passing kernel lets parts be replaced easily, C++ coded, portable, runs on DOS, RCOSjava precursor.
http://cq-pan.cqu.edu.au/david-jones/Projects/rcos/
Topsy
Teachable Operating System: tiny multithreaded messaging microkernel, in ANSI C; protected threads, memory managed, and thread/process control. From undergraduate course on concurrency, device programming, OS concepts. Descriptions, documents, theses, downloads, contacts, links. [Open Source, GPL]
http://www.tik.ee.ethz.ch/~topsy/
NACHOS
Not Another Completely Heuristic Operating System: teaching OS coded in C++ subset, developed at UC Berkeley for OS and Systems Programming classes; now used worldwide. Descriptions, FAQ, port and bug lists, assignments, downloads. [Open Source]
http://www.cs.washington.edu/homes/tom/nachos/
Nachos/486
Port, extension of instructional OS, now stand-alone, protected mode, multithreaded, compiles in FreeBSD to run on i486+, can be relinked without recompile. Descriptions, report (HTML, PDF, PS), download. [Open Source]
http://www.cs.virginia.edu/~bah6f/nachos486/
AcadOS
Academic Operating System, goals: to expose students to more modern ideas than older OSs, to basic OS mechanisms, to OS and language design and prototyping; old version, ideas for new microkernel version.
http://www.cs.uiowa.edu/~jones/acados/
Kaneton Microkernel
Goal: provide all students need to learn OS concepts: basic functions, kernel internals, microkernel architecture, to advanced distributed concepts, and to develop theirs, step by step.
http://www.kaneton.org/
MikeOS
Tool to learn: how simple OSs work, OS development, x86 assembly language; simple code, ample documentation, often add new features on request. Open source.
http://mikeos.berlios.de/
