PlantUML is an open-source tool allowing users to create UML diagrams from a plain text language. Developed by Arnaud Roques
License: Open Source
PlantUML is a powerful tool favored by developers for its text-based diagramming and integration capabilities, while ProcessOn offers a more user-friendly experience with real-time collaboration and cloud access. The choice between the two largely depends on the user's need for advanced features versus ease of use.