GIMP - GIMP is a multiplatform photo manipulation tool.
draw.io - Online diagramming application
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.
Nektony Mac Visio Viewer - Nektony Mac Visio Viewer is a fully featured application for Mac OS X operating systems to view MS Visio files.
VSD Viewer - VSD Viewer is a free utility for easy opening of the Microsoft Office Vision files in an efficient and convenient way.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.