Another open source gem I have been using for a while is called diagrams.net (previously draw.io) which is a great free diagramming tool that I wanted to share with anyone who hasn’t already found it.
It works really well and integrates into Google Drive and many other services providing numerous diagrams right out of the box. I particularly like it because it is really easy to share diagrams from Google Drive.
It has a nice set of UML diagrams which I am used to and also a large variety of other diagrams for all kinds of needs. Here is a simple flow chart for Alien Assimilator showing the game flow.