Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
Whimsical Wireframes - Lightning fast wireframing with real-time collaboration.
Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀
Balsamiq Cloud - Fast, approachable, collaborative wireframing
Platforma - Wireframe library of layouts and UI elements
Justinmind - Define awesome experiences through wireframes & prototypes.