Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀
draw.io - Online diagramming application
Platforma - Wireframe library of layouts and UI elements
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.