Code2Flow - An easy solution to create product flows.
draw.io - Online diagramming application
DIagrams Through Ascii Art - Java command-line tool to convert ASCII diagrams to bitmaps.
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
Flowchart.js - Draw SVG flow charts from simple text representations of the diagrams.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.