Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀
UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
Platforma - Wireframe library of layouts and UI elements
draw.io - Online diagramming application
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.