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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

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.

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

Support Articles

Apple security updates
TCP and UDP ports used by Apple software products

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

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

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.

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.

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).