For some time I have been looking for an on-line tool to create diagrams for a variety of tasks, and I have recently rediscovered draw.io which I am finding very useful for doing simple mindmaps and venn diagrams (which I use in teaching as a basic logic tool). For both of these functions there are specialist tools, but I am finding draw.io easier to use than most - and sufficient for what I want to do.
The programme will connect to a range of cloud storage facilities so you can save to Google Drive, Idrive etc, and it automatically saves changes there. You can then either publish links to your diagrams or embed them in web pages. The embedded files do take a little while to load so my plan is to embed them while I am still developing them (they, then get automatically updated) but will replace them with static versions when I am finished. There are a couple of examples on my website in the approaches section where I wanted to start with a mindmap rather than text.