Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀
draw.io - Online diagramming application
Platforma - Wireframe library of layouts and UI elements
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Craftwork - A collection of User Interface resources made by Craftwork
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.