Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.
Source Wireframe Kit - Library of 537 blocks for desktop and mobile prototyping
Anthony Boyd Graphics - Anthony Boyd Graphics is the fastest-growing platform that offers high-quality and advanced level mockups, textures, 3D models, and UI kits for free to help designers to save time and effort.
Form Wireframe Kit by InVision - A free wireframe kit from InVision
Mr. Mockup - Mr.
Contra Wireframe Kit - Open source wireframe kit for design and prototyping 🚀