Recommended and mentioned products
Scalable, secure, cross-device and enterprise-ready team collaboration tool for distributed teams. Join 2M+ users & 8000+ teams from around the world.
Draw.io is a free and open source web based diagramming and mind mapping software.
Try app.diagrams.net . It's completely free.
Milanote is a note taking app for creative work.
I second Milanote, it is great for gamedev in general as it is also a nice visual board as well as a flow chart! https://milanote.com/.
The Game Design Platform.
Not sure if its what your looking for but there is also machinations.io, which allows you to model and test the actual gameplay.
Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
Inkscape also lets you create flow charts. It's a vector graphics program, but can be used for a lot of stuff.