This is a constantly work-in-progress list of cool project I have come across or closely worked with.
It solves for me the "I remember there was a cool project, but the name completely slipped my mind! Halp!" case.

Sooo,

Make 2D, 3D, VR / AR visualisation

• Mywebar In-browser AR
• Potree WebGL point cloud viewer for large datasets
• Sketchfab A marketplace for 3D models
• ThreeJS A library for making in-browser 3D graphic
• D3JS A library for data visualization

Do machine learning

• Openpilot Open source driver assistance system
• Google Tesseract an OCR library

Emulate

• Puppeteer headless Chrome emulator
• Playwright Cross-browser API automation
• Localstack Local AWS for development

Debug your app

• chrome-remote-interface Chrome Debugging Protocol interface for different environments

Parse something

• Cheerio HTML parser
• Typescript API JS/TS parser
• Espree another JS/TS parser

Compile something

• Pkg Compile your Node app into binary

Highlight syntax

• PrismJS

Create an editor

• DraftJS Medium-like editor framework
• Monaco Editor In-browser source code editor
• React Ace

Develop for mobile

• Flutter

Process some images

• ImageMagic Good article here
• Sharp

Work with a database

• TypeORM
• Prisma
• Minimongo Client-side Mongo backed by IndexedDb

Etc

• Alan create a voice assistant