Libre office draw is wonderful, it can edit pdf docs at ease. It’s crazly powerful compared to paint 3d
UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
draw.io - Online diagramming application
Freddy Feedback - Freddy Feedback is a tool to get feedback from your visitors and customers on anything that you communicate to them on your website.
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.
Userflows - Automatically generate user journeys
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.