excalidraw

A whiteboard that renders everything in a hand-drawn style, which sounds cosmetic and is not. Sketchy diagrams read as provisional, so people argue with the architecture instead of the box alignment. End-to-end encrypted live collaboration, works offline, and the npm package embeds into your own app. MIT licensed and self-hostable. The tradeoff is that it stays deliberately simple: no swimlanes, no auto-layout, no shape libraries at Visio depth. If your diagram needs to survive six months of edits, it will fight you. For the whiteboard stage of a design discussion, nothing else is this fast to reach for.