Domainatrix: Assembly language OS for Intel/AMD x86 architecture made specifically for serving the Web, written originally for the programmer's personal interest and education after moving to the PC platform from his old 8-bit machine. Philosophically similar to V2 OS.
Download Operating System: LSD: By LadSoft; developed years ago and improved since then; planed for redesign to make a viable OS; updates soon. Extant version is good sample of programming standard PC hardware (no PCI) and managing x86 pmode resources. [Copyrighted Freeware]
Hactar: Claims: all new, no equivalent design in extant OSs, an adventure into something really different, an answer to a What if? Runs on 386+, system disk is floppy, kernel size 32Kb; manages 4Gb RAM, 2Gb disk; semi-multitasking; unusual interface. [Open Source, WTOF PL]
HanoiOS: Self booting x86 OS implementation of Towers of Hanoi puzzle, very simple bootstrap loader and kernel; boots, presents shell-like prompt for valid integer input, used by kernel as count of disks to solve, prints list of moves.
PizziOS: One-user, segmented memory OS, written in pure x86 Assembly but may get some C, memory protection via segmentation, FAT-16/32. Main goals: small, fast. Will not be a Unix, but will use block shifting memory management scheme used in some old Unix. Downloads.
PM_SZ_OS: Protected mode, 32-bit OS for i486 CPUs and up, full network stack, supports several filesystems, paging, deferred procedure call support, and virtual multiple screens with different resolution and scrollback. Free download.
RDOS: Multi-process/threaded protected mode OS for all i386+ systems, written in all 16-/32-bit Assembly in TASM, fully PROMable so can port to embedded systems with no keyboard and/or screen, stable and secure via segment protection and paging. [Open Source, GPL]
ScottsNewOS: Runs on i386+ compatible, working GUI even on 4 MB. Coded in Assembly, C. Brief description, download link. [Open Source, GPL]
SkyOS: Open-source, full featured graphical (GUI) 32-bit operating system for x86 386+ architecture; network and Sound Blaster support, mice, video, and many file systems. Website shows requirements, screenshots, status, downloads, documentation, contacts, members, forum, mail list.
SuperDOS: Multitasking, multiuser business OS; small (100K), integrated file system, low maintenance. Main purpose: fast business online transaction processing. Usually used in vertical markets such as transportation. Not a (unrelated to) DOS variant. By Bluebird Systems.
Synapse Operating Services: Protected mode, multitasking support, 32-bit; developed on x86-32, intended to run on x86-64 with some changes; DOS-like, but not fully compatible, use: to run ported DOS programs up to 512K. At SourceForge. [Open Source, BSD]
The Mobius: SourceForge: A few screenshots, details, links.
TriangleOS: 32-bit multitasking, multithreaded OS for x86 386+ CPUs, with GUI, FSs, applications, drivers, all in active development. Site has information, news, documents, screenshots, downloadable releases.
Visopsys: VISual OPerating SYStem: small, fast, home-brewed, open source OS kernel for x86. Includes project background, screenshots, source code and useful links for OS developers. [Open Source, GPL]
XAOS: Runs on i386+, 16-bit real mode, FAT12/16/32 (read-only, LFN support), coded in C++; downloads: binaries, source, utilities, game (tetris source); downloads. [Open Source]