diagrams.net (draw.io)
Online toolFree diagram editor (flowcharts, network diagrams, UML…), online or as a desktop app, saving wherever you choose.
Completely freeMacFree alternatives to Microsoft Visio2 alternatives
Free alternatives to Microsoft Visio, compared on what really matters: what is free, the licence, whether an account is required and the platforms.
Excalidraw
Online toolVirtual whiteboard with a hand-drawn look, for sketches, diagrams and live collaboration.
Limited free planWeb