Mermaid

Knut Sveidqvist started the project in 2014. It supports flowcharts, sequence diagrams, class diagrams, Gantt charts, ER diagrams and a handful more. The point is that the diagram lives in version control as text and can therefore be diffed and reviewed.

GitHub has rendered Mermaid blocks in markdown since 2022, which made it the de facto standard in READMEs and architecture docs. Also popular as an output format from language models, since generating correct text is easier than generating correct SVG.