Code2Flow - An easy solution to create product flows.
asciiflow - Infinite ASCII diagrams, save to Google Drive, resize, freeform draw, and export straight to text/html.
Flowchart.js - Draw SVG flow charts from simple text representations of the diagrams.
DIagrams Through Ascii Art - Java command-line tool to convert ASCII diagrams to bitmaps.
LaTeXDraw - LaTeXDraw is a graphical drawing editor for LaTeX.
draw.io - Online diagramming application