Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
GIMP - GIMP is a multiplatform photo manipulation tool.
Graphity - Graphity is a diagram editor that can be used to quickly and effectively generate drawings and to...
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.
VSD Viewer - VSD Viewer is a free utility for easy opening of the Microsoft Office Vision files in an efficient and convenient way.
draw.io - Online diagramming application