Excalidraw

Excalidraw is a free, open-source whiteboard that lets you sketch diagrams, flowcharts, and wireframes with a friendly hand-drawn look. Its minimal interface keeps you focused, and you can drop in shapes, arrows, and text in seconds.

Drawings are saved locally and can be exported as images or shared through live collaboration links, all without an account. Popular with developers and product teams for quick architecture sketches and brainstorming, it is both privacy-friendly and endlessly handy.