mermaid!

PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams. The generator can be run anywhere within JVM and integrated with various application such as wiki, text editor, IDE, programming language, documentation generator, and others. Mostly based on Graphviz. Can render various diagram syntax such as PlantUML, Dot, and Ditaa.

Graphviz is open source graph visualization software. It has several main graph layout programs. It also has web and interactive graphical interfaces, and auxiliary tools, libraries, and language bindings.

WebGraphviz is Graphviz in the Browser.

blockdiag and its family generate diagram images from simply text file.