PlantUML
Writes UML diagrams as text. Alice -> Bob: Hello becomes a sequence diagram. Started in 2009 by Arnaud Roques and long the standard for diagrams in version-controlled documentation.
It covers considerably more diagram types than its competitors: sequence, class, component, state, activity, use case, deployment, ER, Gantt, mindmap and C4 via a library. Sequence diagrams are drawn by its own engine while most other types use Graphviz for layout. It requires Java, which is the most common objection — it's a jar, not an npm package, and integrations often call a public rendering server you may not want to send your architecture to. Works in IntelliJ, VS Code, Confluence and Doxygen. Mermaid has taken over much of the usage since it renders directly in the browser and is supported by GitHub, but PlantUML remains more complete for actual UML.