Excalidraw

Excalidraw is a collaborative and end-to-end encrypted virtual whiteboard that lets you create sketches and diagrams that appear hand-drawn.

For Obsidian users, there’s the obsidian-excalidraw-plugin which embeds Excalidraw natively. This combination makes it super easy to embed sketches into notes, while automatically keeping a png/svg image up-to-date for easy exporting.