Apple - Mac OS

SpiderNet - Macintosh Online Service
Apple Support - Older Apple software downloads
Low End Mac Low End Mac - Classic Mac OS Downloads and Updates
The Apple Wiki

OpenCore Legacy Patcher for installing more recent macOS versions on hardware that officially does not support those version. GitHub - dortania/OpenCore-Legacy-Patcher.

SheepShaver is an open source PowerPC Apple Macintosh emulator. Using SheepShaver (along with the appropriate ROM image) it is possible to emulate a PowerPC Macintosh computer capable of running Mac OS 7.5.2 through 9.0.4. Builds of SheepShaver are available for Mac OS X, Windows and Linux. See also: http://sheepshaver.cebix.net/SheepShaver.

7402.org - Blog - Sandboxing a third-party macOS app to restrict writing to one folder

Mavericks Forever. Or: How to still keep running on macOS 10.9.
ars technica - OS X 10.9 Mavericks: The Ars Technica Review

GitHub - adespoton/utmconfigs Config files for booting Mac OS 7-9, OS X and macOS on UTM emulator.

MacStrategy is a web site with lots of free (technical) information and news about Apple products with a specific slant to Europe.

Macmaps - Upgrade Paths for Macs running Intel & and Apple Silicon CPUs

OSXDaily - How to Find Comprehensive Mac System Info via Terminal:

system_profiler SPSoftwareDataType SPHardwareDataType

GeeksChalk - How to Download MacOS Installers for New & Old Versions

Apple - Support - Documentaation - Mac
Apple Developer - Documentation Archive
Apple Developer - Documentation

University of Utah - Mac Managers
Marriott Library – Apple Infrastructure Blog Apple Support & Integration in a Higher Education Heterogeneous Environment. Has, among other things, links to the MacAdmins Meeting Archived Presentations and Slides that is held every month.

Support Articles

Apple security updates
Update macOS on Mac
TCP and UDP ports used by Apple software products
Create a bootable installer for macOS
Mac startup key combinations
If your Mac doesn't start up all the way

Mac OS X 10.5 Server - Technical Specifications

OS X Lion Server - Technical Specifications

Documentation - OS X Mavericks has links to the (security) updates for Mavericks.
Download Security Update 2016-004 Mavericks

OS X El Capitan - Technical Specifications
Download OS X El Capitan 10.11.6 Combo Update

macOS High Sierra - Technical Specifications

Xserve - Technical Specifications for G4 Xserve.
Xserve G5 - Technical Specifications
Xserve (2009) - Technical Specifications

Mac OS 9

Apple Support - Mac OS 9: Available Updates dead link, use archive.org - Apple Support - Mac OS 9: Available Updates.
archive.org - Apple - Mac OS 9.2.2: Document and Software
Apple Support - Multicountry directory - Dutch Mac OS 9 and lower updates dead link, use archive.org - Apple Support - Multicountry directory - Dutch Mac OS 9 and lower updates.

Apple Support - Multicountry directory for software relevant to other languages. Links for 9.2.x updates seem to be dead. Use this as a starting point to get the different language versions. Just remove the http://web.archive.org/web/20100917074207/ from the link to the .smi.bin file. Or try here instead for the updates for the US English version.

Pure Mac - Mac OS 9 Updates
Updates for Mac OS 9
OS 9 Forever.com
Macintosharcive.org's mirror of max1zzz.co.uk contains all 9.x.x updates upto 9.2.2 in several language versions.

Archived - Mac OS 9.2.2: Document and Software
OS 9.2.3 is the unofficial update to Mac OS 9.2.2. It adds 17 system menus with 180 new commands to the Finder. Most of them are disk and file utilities. This is NOT an Apple product. By Cliff Huylebroeck. Dead link, use archive.org - OS 9.2.3.

Mac OS Server

Apple - macOS Server User Guide

Apple Support - About macOS Server 5.7.1 and later

The Apple Wiki - Firmware/Mac Server/10.6.x with download links for 10.6.1 to 10.6.8 for Mac OS 10.6 Server.

Apple - Business Essentials One complete subscription that seamlessly brings together device management, 24/7 support, and cloud storage. With Apple Business Essentials, your small business can easily manage every iPhone, iPad, Mac, and Apple TV — every step of the way.

Apple Community - macOS Server Replacement: Calendar, Contacts, Mail, VPN, DNS
Apple Community - End of macOS Server, Alternative ?

GitHub - enzo-zsh/awesome-macos-server A list of free and paid apps to turn your Mac into a server.

7402.org - Blog - Replacement of macOS Server: Calendar, Contacts, and Mail

archive.org - Apple - Mac OS X Server - Resources - Documentation has the links to PDF files for Mac OS 10.6 Snow Leopard Server.
archive.org - Apple - Mac OS X Server - Resources (2009) has links to the 10.5 Leopoard Server PDFs that are also available from archive.org. Found via MacRumors - Forums - OS X Server on PPC.

Searching DuckDuckGo for the following site:URL terms gives links to many of the Mac OS X Server PDF files, but only for 10.4 Tiger.

site:https://images.apple.com/server/docs
site:https://www.apple.com/server/docs

Shaw's Rhapsody Resource Page

Applications

GitHub - phmullins/awesome-macos. A curated list of awesome software for Apple's macOS.
GitHub - ringomdm/mac-admin-tools. Free mac admin tools.

GitHub - BITespresso/createinstalliso. Creates a bootable ISO image from a macOS installer application. This image can then be used to install macOS on a physical or virtual machine.
DiskMaker X is an application built with AppleScript that you can use with many versions of OS X/macOS to build a bootable drive from OS X/macOS installer program (the one you download from the App Store).

Classilla is a free, open source browser for Mac OS 9 and where possible Mac OS 8.6 built on WaMCom, a port of Mozilla to classic Macintosh system. Development has ended in 2021: Old Vintage Computing Research - The final official release of Classilla.
TenFourFox “The world's most advanced web browser. Finely tuned for PowerPC”. Development has ended: TenFourFoxDevelopment - The end of TenFourFox and what I've learned from it.

Marcel Bresink Software-Systeme. Home of:

Tinker Tool
Tinker Tool System
PrefEdit
PropEdit
and others

Cutedge Systems. Home of MailServe.

GrowlyBird Software. Home of the free apps Growly Draw, Growly Notes, Growly Write, Growly Calendar, Growly Photo, and several other productivity applications and games.

Andrew C. Stone with several iOS and macOS applications.

Calendar and Contacts Server the basis of the Calendar and Contacts server in Mac OS X Server. GitHub - apple/ccs-calendarserver (archived), and GitHub - JohnPritchard/ccs-calendarserver a slightly updated fork. See also: stewart a. | aka firewing1 - Installing the Darwin Calendar Server 2.4 on Fedora 13 or Fedora 14.

Software/Package managers

The MacPorts Project is an open-source community initiative to design an easy-to-use system for compiling, installing, and upgrading either command-line, X11 or Aqua based open-source software on the Mac operating system. To that end we provide the command-line driven MacPorts software package under a 3-Clause BSD License, and through it easy access to thousands of ports that greatly simplify the task of compiling and installing open-source software on your Mac.

Homebrew The Missing Package Manager for macOS (or Linux).