PlantUML Specification
The Specification for PlantUML
Share your knowledge by contributing to this by editing the online wiki
Best of PlantUML
Highlighting the best of the PlantUML community diagrams
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PlantUML Forum
Ask questions here to the PlantUML community
Or share your knowledge by answering questions from other users
Hitchhiker's Guide
Unleash the power of PlantUML icon diagrams with this step-by-step user-centric guide
Real World Plantuml
A great site by @yfutuyama who created the website Real-World-PlantUML to solve these issues: