Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Graphity - Graphity is a diagram editor that can be used to quickly and effectively generate drawings and to...
GeneVenn - GeneVenn is an online diagram editor that allows the users to enter the list and may use the text area in order to assign letters on the list in the diagram.
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.
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.