Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
Code2Flow - An easy solution to create product flows.
LaTeXDraw - LaTeXDraw is a graphical drawing editor for LaTeX.
asciiflow - Infinite ASCII diagrams, save to Google Drive, resize, freeform draw, and export straight to text/html.
Ipe - Ipe is a drawing editor for creating figures in PDF or (encapsulated) Postscript format.
Flowchart.js - Draw SVG flow charts from simple text representations of the diagrams.