Wireflow - Create beautiful flowcharts
draw.io - Online diagramming application
Platforma Flowcharts - Collection of cards for website and app prototyping
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.
Whimsical - The visual workspace for teams.
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.