Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀
Invision - Prototyping and collaboration for design teams
Platforma - Wireframe library of layouts and UI elements
Zeplin - Collaboration app for UI designers & frontend developers