Excalidraw

Excalidraw is a browser-based collaborative whiteboard for hand-drawn diagrams, wireframes and sketches. It is open source, opens instantly without an account, and supports real-time multiplayer. The deliberate hand-drawn aesthetic is the point rather than a limitation: a rough-looking diagram reads as provisional, so people critique the idea instead of the alignment.

It is in active use here. We use it for architecture sketches, sequence diagrams and live whiteboarding on calls. Most of what we draw is discarded within the hour, and it is fast enough that drawing does not interrupt the conversation. When a diagram needs to survive, we export it and check it into the repository.

FigJam is the better call if your team already works in Figma and wants comments, voting and workshop tooling around the canvas. Miro is stronger for large facilitated sessions with many participants. tldraw is a close open-source cousin worth looking at if you want to embed a canvas in your own product. Whimsical and Lucidchart make more sense for polished, presentable diagrams. Mermaid is the option when the diagram should live in version control as text. Excalidraw is the right call for fast, disposable thinking. It is the wrong call when the artefact has to look finished for an external audience.