diagrams.net – Diagram with anyone, anywhere

diagrams.net (formerly called Draw.io) is a completely free, open-source, cross-platform and stand-alone flowchart drawing application that can be used as a browser-based web application, desktop application and plug-in for some other software. That's to say, you can use its web edition directly in a web browser (no registration required, no limitations, no catches), or use its desktop edition offline under Windows, Mac or Linux OS.