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--. You Would also find info related to FreeBSD, homepage, thinkpad, packet and much more.

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]

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.

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]

SunriseOS
Tiny operating system written in Assembly, via NASM, by ICT student. Meant for only fun and learning. [Open Source]

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.

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]

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]

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] You Would also find info related to Visual Opal and much more.

Instinct Operating System
A command-line OS written in x86 assembly, designed as a learning tool for college students and amateur programmers. Download available. [Open Source]

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]

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]

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]