Violet is a UML editor with these benefits: Very easy to learn and use. Draws nice-looking diagrams. Completely free. Cross-platform. Violet is intended for developers, students, teachers, and authors who need to produce simple UML diagrams quickly.
Want to contribute ? Source code is hosted on github : https://github.com/violetumleditor/violetumleditor
I installed it but the help menu of the program is not active.
Light and easy product. Best used for quick prototyping.