PlantUML
Generate diagrams from textual description
...Run (or have your software call) PlantUML, using sequenceDiagram.txt as input. The output is an image, which either appears in the other software, or is written to an image file on disk. Diagrams are defined using a simple and intuitive language. (see PlantUML Language Reference Guide). Images can be generated in PNG, in SVG or in LaTeX format. It is also possible to generate ASCII art diagrams (only for sequence diagrams).