Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
Diagrams Haskell package - Diagrams is a powerful, flexible, declarative domain-specific language for creating vector...
LaTeXDraw - LaTeXDraw is a graphical drawing editor for LaTeX.
Ipe - Ipe is a drawing editor for creating figures in PDF or (encapsulated) Postscript format.
Diagrammix - Tool for efficiently building attractive diagrams.
Graphviz - Graphviz is open source graph visualization software. It has several main graph layout programs.