Diagram

Flowgorithm is a free programming language aimed at beginners based on simple graphical flowcharts. It supports more than 18 languages.

I've been using Canva for mind maps, business planning charts and it works quite well.

Open Source Free Form Data Organizer that can be used for spreadsheets, mind mappers, outliners, PIMs, text editors and small databases.

BigHairyGoal helps me gain clarity in many different subjects. I love having an overall view of what needs to be done and what has been done

Free and open source modelling tool for creating ArchiMate models and sketches written in Java.

"Ideaflip is a great mind mapping tool for capturing ideas and building mind maps, though it lacks analytics tools that some teams may require" according to PC Magazine.

Has a lot of features

Great

Easy to use, can generate change script, and has a very good integration with MySQL (incl. PHP app generator). And it's really free (no limits).

After an initial learning curve, you can very quickly draw diagrams and export these.

Gleek is an online tool that supports Flowcharts, Entity Relationship diagrams and UML diagrams.

A lightweight basic drawing application in the Google Drive Suit. Very easy to use for quick drawings and diagrams. Exports to pdf, png, jpg, or svg.

Draw.io is a free web based diagramming and mind mapping software.

Free architecture design, with templates for database, serverless apps, machine learning, compute app, containerized app, etc.

Free Modeling Tool available as Open Source and in a paid version.