Apple
 System .85 / Finder 1.0
 The Mac 512K
 Image Gallery - MacOS
 What's New in Mac OS 8?
 The Mothership! - early Mac and Lisa archive
 Apple's Twiggy Disks
 Apple Lisa
 Apple Lisa 2/10
 CrazyChucks.com Apple Lisa Page
 Lisa Emulator Project
 Tom Stepleton's Apple Lisa Photos
 Apple A/UX
 Intro to NEXTSTEP
 Archives NeXT
 Apple - Mac OS X
 Aqua Human Interface Guidelines
 Apple-History
 XGS/32 - The Official Home Page

Amiga
 Workbench Nostalgia
 Amiga Screen Shots (Danish site)
 HAAGE & PARTNER Computer [ AmigaOS 3.9 ]
 Official AmigaOS Portal
 AmigaOS 3.5 Manual - DOS
 AROS - The Amiga Research OS
 MorphOS Home Page

Apollo
 Apollo Photo Gallery
 Apollo DomainOS info and archive
 Mike's Apollo DN3500 Page

AtheOS
 AtheOS - free alternate operating system
 Introduction & Review of AtheOS 0.3.7 - OSNews.com

BeOS
 First Look at YellowTABs Zeta
 BeOS 5 User's Guide : Table of Contents
 Haiku Operating System

Tandy Deskmate
 Tandy's deskmate (GUI)
 Tandy 1000 TL/2
 Tandy Deskmate Screen Shots

GEM
 Screenshots of GEM
 GEM Workshop
 Digital Research's GEM
 GEM-related news and development
 Gem Programs
 NeoDesk - A revolutionary replacement for the built-in desktop
 ATARI.ORG / Pathfinder : ST : Emulation
 What is TOS?
 The Little Green Desktop - The Real Atari ST!

GEOS
 Geos Web Page
 GEOS Download
 Sean Huxter - Commodore 64 Work
 Remember Q-Link
 History of GEOS for Commodore Computers
 PC/GEOS History
 Welcome to Breadbox
 Unofficial GEOS Web Page

Linux / Unix
 KDE - Screenshots
 GNOME - See GNOME in action
 Enlightenment Home Page
 Official AfterStep Site: Themes Page
 icewm - Home Page
 The MGR Window System
 More Window Managers for X
 WineHQ
 Unix Desktop Environment-Project
 DECwindows User's Guide
 IRIX Interactive User Interface Guidelines
 Solaris 10 Build 69

OS/2
 OS/2 V1.0 (non graphical) screen shots
 OS/2 V1.1 and 1.2 screen shots
 OS/2 V1.3 screen shots
 OS/2 2.1
 OS/2 Warp, PowerPC Edition
 OS/2 Warp 4 Desktop Tour
 eComStation Screenshot Gallery
 Project Odin
 OS/2 Warp History

Perq
 PERQ Workstations
 FAQ for PERQ Graphics Workstations
 The PERQ Graphics Workstation WWW Page and Archive
 DigiBarn: PERQ T2, Ad and other PERQs
 Perq Single User Systems Overview
 Brochures for Apple II and Perq (1979)
 Perq +246perq+
 Perq Floppy Disk Images
 Perq Documentation

QNX
 QNX Photon microGUI
 Get QNX Realtime Platform

RISC OS
 RISCOSltd
 IYONIX pc
 Archie - Acorn Archimedes Emulator
 History of Acorn & RISC OS
 The Acorn Emulation Page - RISC OS 2
 The Acorn Emulation Page - Arthur OS
 Acorn RISC OS
 Technical history of Acorn

Visi On
 Visi On History
 Computer Chronicles - Video of Visi On and Apple Lisa
 VisiCorp speaks.

Windows
 ReactOS - The future replacement for Microsoft Windows
 98lite: Windows98 *without* IE
 Microsoft® Bob™ version 1.0 for Windows®
 TechTV | Remembering Microsoft Bob (Video)
 Innovative Systems I have Used - HP New Wave
 HP NewWave Desktop Demo
 Packard Bell Navigator
 Silver Wolf Desktop, Overview
 Calmira home page
 Die Windows History (german)
 Reviewers Guide to Chicago Beta-1/Early Windows 95 (Word fmt)
 Paul Thurrott's SuperSite for Windows: Reviews

Xerox
 Xerox Alto Operating System And Alto Applications
 The Xerox Alto Computer
 Al's Xerox Workstation Collection - information about Xerox hardware
 DigiBarn: A Brief Telling of the story of the Elixir Products
 A Decade of Research @ Xerox PARC
 Xerox Alto computer
 SmallStar (Bitmapped screen shot of Xerox Star)
 Xerox Star Historical Documents
 Xerox Star Research
 The Xerox Mouse
 Xerox Mesa System Emulator

Future GUIs and previews
 Task Gallery
 ROOMS 3D
 3DNA
 Croquet Project
 Doom as an Interface for Process Management
 Project Looking Glass

Misc
 Doug Engelbart 1968 Demo
 PCmuseum - screenshots of misc old OSes
 MenuetOS.org
 Symphony OS
 SkyOS -- Screenshots
 Plan 9 and its window manager "8 1/2"
 The Microwindows Project
 BTRON-based Products
 Tilcon Software Limited - Graphics Editor
 ETH Oberon System / Gadgets Desktop
 Pascal Graphical Environment
 Historia Graficznych Systemów Operacyjnych

Downloads

Downloadable GUI programs.
TechKnight.Com | Endangered Software Archive Windows 1.01, 1.03, 2.03 and MS-DOS 1.10.
Windows 1.01 1.4mb boot disk - A self contained pre-configured easy to make disk that boots to Windows 1.01 on any PC.  Take the Win 1.01 challenge!
VisiCorp Visi On Bootable 1.4 meg disk image - includes the tutorial and the word processor (this requires a Mouse Systems PC Mouse compatible serial mouse to run)
A preconfigured zip file of Visi On with executables for XT and AT computers. This includes Graph, however Graph will only run on an XT. also the Original unaltered files from the install disks.
GEM World Download GEM 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 2.0, 2.1, 2.2, 3.0, 3.01d, Viewmax, and GEM source code!
Free QNX Demo Disks: QNX OS + Photon GUI + PPP or LAN  + Voyager browser on a single 1.44 meg disk!!!
BeOS V5.0 Personal Edition - a FREE version of BeOS 5 that can be installed from Windows.
QNX 1.44 meg floppy demo disks: Modem version Network card version
Get QNX Realtime Platform - a FREE version of QNX that can be installed from Windows.
Download Sun Solaris - Solaris  .ISOs
Basilisk II - A free Macintosh emulator (emulates color 68K Macintoshes). You will need one of the Mac Quadra or 1024k ROMs
vMac - A free Macintosh emulator (emulates a MacPlus). You will also need a Macintosh 128k ROM also old versions of MacOS
The Archive (Lisa software including Microsoft Xenix)
MacOS 7.5.3 and the update to MacOS 7.5.5 - Straight from Apple's web site. And because I get asked so much:  Installing Mac OS 7.5.3 on Basilisk II and  CFM-68K Runtime Enabler 4.0 for Netscape and Archived versions of Netscape for 68k Mac Also Appearance manager to give MacOS 7.5.5 the MacOS 8 3-d look.
Apple IIGS/OS 6.0.1 - Straight from Apple's web site
XGS/32 - The Official Home Page - An Apple IIgs emulator for Windows.
The Apple II Desktop - a Macintosh like desktop for the Apple II series (Now includes floppy disk images).
ApplePC 2.52b emulator - an Apple II emulator that can run the Apple II desktop. Also needs the roms.
The Little Green Desktop - The Real Atari ST! - Atari ST info, emulator & roms.
Breadbox Ensemble Lite
Geos for the Apple II
GEOS for the C64 and C128
The eComStation Demo CD - A free demo CD of the latest and greatest version of OS/2
OS22Demo.zip - An animated slideshow demo of OS2 Version 2.0
OS2demo.zip - An animated slideshow demo of OS2 Warp V3
Acorn Archimedes Emulators  - Emulates RISC OS 2.0 - 3.11
DESQview and DESQview/X
OS/2 Workplace Shell for MS-Windows 3.1 (does not work on Windows 9x)
Prodemo.zip - An animated slide show of GeoWorks Pro (PC-GEOS) Version 1.2
NewShell for Windows NT 3.51 - add the Windows 95 shell to NT 3.51
NewShell 2 for Windows NT 3.51 - same as first version except fixes a couple of compatibility issues.
PubTech File Organizer 2.11 for Windows 2.x
PubTech File Organizer 3.10 for Windows 3.0
HP NewWave Desktop Demo
Calmira Desktop for Windows 3.1
Screen Savers for Windows 3.1
Allgemeine Downloads (has MS-bob)
GlobalView 2.1 for Windows 3.1, 95, 98
Power Menu 4.11 (A text based Menu/File manager for DOS)
Tandy Presents the World of Multimedia (made with the sounds from Windows 3.0 MME)