The following is offered as Quick Reference to Add-ons (Official or otherwise), Community Tools, Projects, Tutorials, or similar.

I don?t want to link to every Useful Thread out there but if I've missed anything obvious, please let me know.

General:
AGK Home, Bonus Tutorials, Online Help: [url=https://forum.thegamecreators.com/outbound?url=https%3A%2F%2Fwww.appgamekit.com%2Fdocumentation%2Fhome.html]Classic[/url] and [url=https://forum.thegamecreators.com/outbound?url=https%3A%2F%2Fwww.appgamekit.com%2Fdocumentation-studio%2Fhome.html">Studio,
Studio User Guide (PDF)
AGK Players: iOS
AGK on STEAM: Classic Community, Install Previous Versions
TGC on itch.io, Newest games Made with AGK, and the Official AGK Jams Thread
TGC on GitHub, AGK Repo 2024+, Retired but still useful: AGK/S Issues
AGK on YouTube, Tutorial Playlists: Quick Tips for Beginners, Learn to Code with AGK, Creating a Physics Game in Studio
AGK Discord Server (https://discord.gg/vx46Ddb), Unofficial French Server

Manual
Visual Studio Code, see attached Studio Extension (WIN-only) Set-up Guide HERE.
Java Development Kit (JDK) - Required for Android Export, (manual) Setup Guide, if necessary.
DirectX End-User Runtimes (June 2010).

Official Add-Ons, related Threads, and Search:
AGK Games Packs Two - FREE! Contains Source Code!
AGK for Java & Kotlin, Search: Java
AGK Mobile, Mobile
AGK Particle Editor, Thread, Search: Particle Editor
AGK for Python, Python
AGK for Raspberry Pi, Board, Search: Raspberry Pi
AGK Serial Plugin, Arduino Tutorial, Search: Serial
AGK Shader Pack, Thread, Search: Shader Pack
AGK Sharp, Sharp
AGK Visual Editor, Thread, Search: Visual Editor
AGK VR, VR
GameGuru Loader, Official User Guide (PDF), YouTube [url=https://forum.thegamecreators.com/outbound?url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3D0BFmtDTkW9A%26list%3DPLrTSqpw8zXL4n3RgVZd2sXuFKDAH4DRmn%26pp%3DiAQB]Playlist[/url], Search: [url=https://forum.thegamecreators.com/outbound?url=https%3A%2F%2Fforum.thegamecreators.com%2Fsearch%3Fq%3Dgameguru%26type%3Dthreads%26boardid%3D41">GameGuru

Resources, Reference and Further Reading:
3D: Blender to AGK (Tutorial), Export .x from Blender (Guide) Blender import utility, Blender 4 Tutorials (YouTube) by Digital Skills
Asset Protection: Vishnu Studio, WadPacker, AES256 Encryption
Advanced math functions by Jack
AGK > HTML Knowledge Base
Box2D Physics: Website & GitHub, Code: Top Down Driving
Bullet3D Physics: AGK 3D Physics Demos (Source), Website, Code: Kart Physics
AGK Code Snippets (filter search: [AGKS]), and CodeBase Entries
List of Community Developed Plugins including Steam API and FileExplore
[How To] make a Plugin with PureBasic by PTC
Hands On AppGameKit Studio (PDF Books) by Digital-Skills, Download Free Chapters
Shaders: EVOLVED's DBPro Shaders ported to AGK, Community Shaders Project, Shader Suite, The Book of Shaders
PurpleToken.com - Online High Scores service provided by Phaelax
Tilemap Tutorial by Phaelax
Community Toolkit including DL)
Font Image Tools: BMFC, UO BitmapFontMaker, .PSD Template
Small Game Templates (Source), Announcement, & Info (PDF)
Setting up the Display for Multiple Devices (Percentage System Tutorial) by Scraggle
Useful Community Functions Project
Useful Tools, Effects and Snippets (& lots more)
ZInput Controller Input System by Rudolpho (WIN DLL)

@Virtual Nomad

Thanks for posting

Excellent idea

Thank you, very useful!

Thanks a lot!

AGK Discord Channel added.

the official 3d physics demos are now attached to the first post for more-permanent storage and easier reference.

from a recent inquiry, i've added reference to Manual, Help Doc @ TGC GitHub to the first post

AGK Steam links added to first post.

added The Book of Shaders as the go-to resource for shader development.

first post updated to add 2 blender guides under 3D.

Links added for:
AGK Players: iOS

Thank for this, and the updating of it.

added PartTimeCoder's [How To] make a plugin with PureBasic

Added Blender import utility

Studio User Guide (PDF) added

Thank you for this update

Advanced math functions by Jack
...added to the queue. +120 commands!

Unofficial French Discord Server added courtesy of Blendman.

added TGC on itch.io, Newest games Made with AGK, and the Official AGK Jams Thread to first post where official presence has become active and TGC plans ongoing jams, there.