Excalidraw is a free, open-source whiteboarding tool designed for quick sketches and informal diagramming. It has an intentional hand-drawn aesthetic—diagrams look like they're sketched on a whiteboard, not designed in Figma. You can draw shapes (rectangles, circles, diamonds), add text, connect elements, and export as PNG, SVG, or Excalidraw's JSON format. Real-time collaboration is built-in: share a link and teammates can draw together simultaneously. No signup required. Perfect for brainstorming sessions, architecture discussions, and rapid prototyping.