ShawnOS  
   Sure as Hell Ain't Windows Ninety five, the corner cutting OS due to the original schedule. Always intended more as a learning experience for author and others than as a practical, useful OS. Everything done as simply as possible. [Open Source, GPL]
   http://www.overwhelmed.org/shawn/

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/

EduOS  
   Operating system for the IA-32 (80386+) processors. For education purposes only, includes full source code, under active development. Needs Linux or BeOS to be compiled. [Open Source]
   http://inf4serv.rug.ac.be/~fcorneli/eduos/

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
   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
   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, dow
   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/

FDOS  
   Essay and OS for learning, so people who want to know more of technicals in PCs can find interesting answers presented easily; FAT 12 file, directory support; 100% NASM, documents, website in English, German; (c) Stefan Tappertzhofen. [Open Source, GPL]
   http://www.visual-opal.de/

Doug's Simple Operating System: DS/OS  
   Good way to learn OS programming: balanced amount of functionality for learning: not too much to overwhelm beginners, yet some advanced features; pure 8086 Assembly (A86 assembler), incompatible with all other OSs. [Open Source]
   http://bioradiation.net/~dsos/

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/