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
No UML 2.0 support (tested sequence diagram). Very bare-bone. Completely unintuitive interface (try adding, moving elements).
Easy to use, works nice on Linux.
It is not backward compatible (does not read) .violet files created with previous versions.