draw.io - Online diagramming application
UMLGraph - UMLGraph is a professional automated drawing tool that allows the designers the declarative specification and drawing of UML class and sequence diagram.
Nektony Mac Visio Viewer - Nektony Mac Visio Viewer is a fully featured application for Mac OS X operating systems to view MS Visio files.
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Oqto Diagram - Oqto Diagram is a powerful diagram creation and editing software that allows the users to make drawing on tablets and smartphones.