Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Platforma - Wireframe library of layouts and UI elements
draw.io - Online diagramming application
iOS Wireframing UX Kit - An easy, speedy way to wireframe for iOS
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.