Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
Spline - Design tool for 3d web experiences
Platforma - Wireframe library of layouts and UI elements
Vectary - Vectary is a free, online 3D modeling tool and sharing platform.
Form Wireframe Kit by InVision - A free wireframe kit from InVision
Kaedim - Use the Kaedim Platform to automatically transform your 2D art, sketches and photos in 3D.