Mockuper - Instant Mockup Generator
Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
Sketchize - Ready to print wireframing sketch sheets 📱
Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀
Mediamodifier - Create beautiful designs and product mockups in seconds.
Platforma - Wireframe library of layouts and UI elements