Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
Form Wireframe Kit by InVision - A free wireframe kit from InVision
draw.io - Online diagramming application
Contra Wireframe Kit - Open source wireframe kit for design and 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.