Libre office draw is wonderful, it can edit pdf docs at ease. It’s crazly powerful compared to paint 3d
draw.io - Online diagramming application
Nektony Mac Visio Viewer - Nektony Mac Visio Viewer is a fully featured application for Mac OS X operating systems to view MS Visio files.
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.
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
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.