Tools for making static diagrams:
- Graphviz
-
programmatically create graph diagrams
- TikZ
-
programmatically draw lines and shapes
- Dia
-
simple but older app to draw diagrams
- Inkscape
-
nice desktop app to draw diagrams
- diagrams.net
-
fancy online diagram-drawing app
- Asciiflow
-
online app to make ascii-art diagrams