Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
LaTeXDraw - LaTeXDraw is a graphical drawing editor for LaTeX.
SVGO - Tool for optimizing SVG files
Ipe - Ipe is a drawing editor for creating figures in PDF or (encapsulated) Postscript format.
SVGminify.com - Removes superfluous information reducing the size of SVG files
Graphviz - Graphviz is open source graph visualization software. It has several main graph layout programs.