draw.io
Powerful, easy-to-use, ugly as fuck. But cheap and open!
Diagramming
Lucid visual collaboration suite: Log in
Super powerful, easy-to-use, and pretty! Expensive.
Flowcharts Syntax | Mermaid
Useful for so many kinds of documents and it's text-based so it's very transportable. Can be tricky to learn the syntax at first. Not good if you're picky about arrangement/look-and-feel.
FigJam
Solid all-rounder. Optimized for collaboration, but useful for creating permanent docs, too.