Ask HN: A modern-looking UML diagram tool?
I use PlantUML[1] for my UML diagrams.  - It uses a human readable text-based file format renderable via the PlantUML jar. Friendly to CLI and git.
  - The diagrams are stylable, should you wish to style them. 
  - There's a PlantUML Integration IntelliJ plugin that's easy to use for preview/rendering[2]
  - Overall simple to use, but I imagine it can as robust as you want it to be. For example, the IntelliJ plugin Sketch.it automatically generates PlantUML diagrams from Java source code[3], and the source code for how it works is available on BitBucket if you want to know how it works[4]

I've seen other people suggest Mermaid.JS[5] before, but I haven't used it so I can't say how it is.  [1]: http://plantuml.com
  [2]: https://plugins.jetbrains.com/plugin/7017-plantuml-integration/
  [3]: https://plugins.jetbrains.com/plugin/10387-sketch-it-
  [4]: https://bitbucket.org/pmesmeur/sketch.it/src/develop/
  [5]: https://mermaidjs.github.io