WhiteStarUML - WhiteStarUML is a fork of StarUML with an intent to revive its Delphi code base by updating code to...
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Eclipse Papyrus - Eclipse Papyrus is an industrial-grade open source Model-Based Engineering tool. Eclipse Papyrus has notably been used successfuly in industrial projects and is the base platform for several industrial modeling tools.
draw.io - Online diagramming application
StarUML - StarUML
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.