draw.io (diagrams.net)

draw.io is the Swiss Army knife of diagramming — it covers more diagram types than any single competitor, from flowcharts and org charts to UML, ER diagrams, network topologies, and BPMN. It works in your browser or as a desktop app with full offline support. The Google Drive, OneDrive, and GitHub integrations mean your diagrams live wherever your documents live. What makes draw.io genuinely useful is its zero-friction approach: open it, draw something, save it. No account required, no premium features gated, no learning curve. The template library is massive and the shape libraries cover virtually every diagram standard. It's not the prettiest diagram tool, and it doesn't have real-time collaboration. But when you need to create a professional-looking diagram quickly — especially one that needs to follow a specific standard — draw.io is the tool that never lets you down.

Good first choice if you want a practical tool without spending the afternoon reading developer docs.