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