Tiptap
Headless rich-text editor built on ProseMirror. The most common 2024 choice for Notion-like WYSIWYG editors. Open source, commercial pro version.
German heroes behind it: Hans Pagel + Philipp Kühn. Extensible via a plugin system. Supports images, tables, collaboration (Yjs integration), markdown shortcuts, slash commands. Competitors: Slate.js (more hands-on), Lexical (Meta, 2022), Quill (classic, less flexible), CKEditor (old enterprise), Plate (built on Slate). Tiptap Cloud sells the hosted collaboration backend.