diagrams.net (draw.io)
Online toolFree diagram editor (flowcharts, network diagrams, UML…), online or as a desktop app, saving wherever you choose.
Completely freeMacDiagrams & whiteboards3 resources
Flowcharts, diagrams and collaborative whiteboards, in your browser.
Excalidraw
Online toolVirtual whiteboard with a hand-drawn look, for sketches, diagrams and live collaboration.
Limited free planWeb
tldraw
Online toolFree online whiteboard for sketches, diagrams and quick collaborative drawings.
Completely freeWeb