Diagrams.net (old name Draw.io) is free online diagram software for making flowcharts, process diagrams, charts, UML and network diagrams. If offers more than dozens of templates. You can create drawings using Diagrams in any modern web browser or using the Diagrams app for Mac, PC, Linux, and as a Google Chrome Extension.
OmniGraffle is another tool to create diagrams and different visual designs. Available for MacOs and iOS only but a user friendly tool.
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