draw.io - Online diagramming application
Isometric - Create beautiful, abstract works of art & optical illusions
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.
Handy Menu for Sketch - Quick access to Sketch App plugins through contextual menu
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.