tldraw
Infinite canvas whiteboard app + SDK. Open source. Famous 2023 "tldraw makes real" demo: draw a UI → get a working webpage via GPT-4 Vision.
Steve Ruiz built it, same author as perfect-freehand. The SDK (tldraw library) embeds the canvas in your app — used by Mozilla, Vercel, Hugging Face Spaces. Multiplayer via Yjs or its own sync engine. Competitors: Excalidraw (simpler + iconic sketch look), tldraw is more feature-rich. Vibrant developer community around sync, AI integrations.