The website diagrams.net is a quite popular online tool for creating simple and non-standardized diagrams.

  • πŸ’΅ It's free
  • 🐣 It doesn't require much knowledge
  • πŸ”‘ It doesn't require an account
  • 🌍 We do not have to install anything

The main disadvantage is that it doesn't enforce a standard (ex: UML).

Without useful and clear explanations (which is usually a complex task), everyone may understand your diagrams in a different way.

It's not appropriate to use this for complex diagrams.