Libre office draw is wonderful, it can edit pdf docs at ease. It’s crazly powerful compared to paint 3d
PGF and TikZ - PGF/TikZ is a tandem of languages for producing vector graphics from a geometric/algebraic...
draw.io - Online diagramming application
Diagrams Haskell package - Diagrams is a powerful, flexible, declarative domain-specific language for creating vector...
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.
LaTeXDraw - LaTeXDraw is a graphical drawing editor for LaTeX.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.