umlet.com
UMLet is a free, open-source UML tool with a simple user interface for drawing UML diagrams. Sequence and activity diagrams can be created from plain text, and diagrams can be exported to eps, pdf, jpg, svg and the clipboard. It is available as a stand-alone app for Windows, macOS and Linux, as an Eclipse plugin, as the UMLetino web app and as a Visual Studio Code extension.