Edit UML and non-UML diagrams saving as SVG or PNG, with PlantUml
PlantUML is a free program you can use to edit UML and non-UML diagrams using descriptions.
The program supports Sequence, Usecase, Class, Activity, Component, State, Object, Deployment, Timing, and also non-UML: Wireframe, Archimate, SDL, Ditaa, Gantt, Mathematic (AsciiMath, JLaTeXMath).
The interface is simple and easy to use. You can create diagrams using a descriptive language or import your data from C, HTML, CPP, Java, plain text, and more, saving them as SVG or PNG.
Note that you need the Java environment to run PlantUML.