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.

NCH Software's tools are famous for compact file size and high efficiency. ClickCharts is a tiny and handy app that can help you easily make simple and intuitive flow charts (including the creation of various diagrams or mind maps), so as to clearly show your thinking, planning to others.