tldraw

tldraw ships a React component that gives you a full canvas out of the box: shapes, tools, selection and transform handles, copy/paste, undo/redo, cross-tab sync, theming, and an extensible shape/tool system. Its optional sync service adds multiplayer with live cursors and presence, persistence, and performance tuning. The team also runs tldraw.com, a free hosted whiteboard, and a downloadable offline app.

Why it's a discovery: it's the fastest way to add a real canvas to a product instead of building one, and it's open source under a community license (a small watermark unless you buy a licence). It's become a common building block for AI "generative UI" demos.

Best for: product teams adding whiteboarding, diagramming, or spatial canvases to their app.