Excalidraw

A virtual whiteboard for sketching diagrams, architecture drawings, and flowcharts in a deliberately hand-drawn style that stays readable when it turns out to be wrong. Multiple people can edit the same canvas at once, shapes snap to a grid, libraries hold reusable elements, and scenes export to SVG, PNG, or a self-contained HTML file.

Engineers, designers, and facilitators use it to sketch ideas quickly, plan systems, and workshop around a shared board.

Self-hosting keeps internal diagrams out of third-party storage; since scenes are simple JSON files, keeping and restoring copies is easy.