SpiderFoot - Open source intelligence (OSINT) automation tool.
draw.io - Online diagramming application
Sherlock - Sherlock provides insights on user engagement for product makers.
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
sn0int - sn0int is a semi-automatic OSINT framework and package manager
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.