PlantUML

A tool where you write a simple text description and it draws a diagram from it. It supports all UML diagrams (sequence, class, activity, state, component, deployment) and many other types such as Gantt charts, mind maps, network diagrams, ER diagrams and org charts. The advantage: the diagram is stored as a text file, so it can be managed in Git, its changes reviewed and embedded in documentation. It runs on Java and uses Graphviz for some diagram types.