Playscii - Full-featured ASCII art and graphic editor and game creation tool
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
draw.io - Online diagramming application
Graphity - Graphity is a diagram editor that can be used to quickly and effectively generate drawings and to...
UMLGraph - UMLGraph is a professional automated drawing tool that allows the designers the declarative specification and drawing of UML class and sequence 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.