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
   http://anywherebb.com/hactar.html

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.
   http://www.kernelthread.com/hanoi/html/tos-x86.html

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. Downloa
   http://www.patmedia.net/eaj.pizzi/pizzios/

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, G
   http://www.rdos.net/

ScottsNewOS
   Runs on i386+ compatible, working GUI even on 4 MB. Coded in Assembly, C. Brief description, download link. [Open Source, GPL]
   http://scottsnewos.sourceforge.net/

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]
   http://sourceforge.net/projects/comos/

The Mobius: SourceForge
   A few screenshots, details, links.
   http://mobius.sourceforge.net/

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.
   http://members.chello.nl/w.cools/

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]
   http://www.visopsys.org/

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]
   http://www.mirzoyan.com/xaos/