PlantUML
Diagramming · Free · since 2009 · open source
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PlantUML is an open-source tool that creates UML diagrams from plain text. It supports class diagrams, sequence diagrams, use case diagrams, activity diagrams, and many more UML types. It integrates with IDEs, CI/CD pipelines, and documentation tools.
pros
- Most comprehensive UML support of any diagramming tool
- Text-based format integrates with version control
- Wide IDE support (VS Code, IntelliJ, Eclipse)
- Active community with 15+ years of development
cons
- Steep learning curve for the text syntax
- Output style is functional but not aesthetically modern
- Layout control is limited — auto-layout can produce awkward results
- Less suitable for non-UML diagram types
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