A big ol' list of GameMaker libraries, extensions, tools etc.
Come chat about GameMaker on Juju's Discord server
If you'd like to add a link to this list, please make a pull request.
Fix GML being recognised as Yacc on GitHub
GM Core (Buncha libraries rolled into one)
Gleb Tsereteli's GameMaker 2.3 Resources
Bronze Box - 3d world models from a 2d grid
Video Player for Windows and Ubuntu
ShaderToy to GameMaker Converter
Angel Engine Snips (Sprite Playback)
NekoPresence (Discord Rich Presence)
Crispy (Unit Testing Framework)
Another Unit Testing Framework
ThunderTracker (Sprite Attachment Points)
Building GameMaker Games from Commandline
Three Mice In A Trenchcoat Source
Room Editor Friendly WarpSystem
Drag and Drop Files Into GameMaker Games
Cartoon Chickens - Cel Shading in GameMaker
Danmaku Project - Bullet Hell Library
Small Pentapop Localization tool
Luminous Chicken - 3D Lighting Shaders for GameMaker
Various Extensions including border toggle, dialog boxes, shell execution, file manager, WebM Player, file dropper etc.
(Editor's note: we should copy https://tinytools.directory/ and/or https://notabug.org/Calinou/awesome-gamedev)