avocode - Centralize design collaboration, version control, prototyping, feedback, and developer hand-off with Avocode. Work on any design, anywhere, with anyone. Sign up free 👉 https://avocode.com/.
Invision - Prototyping and collaboration for design teams
Zeplin - Collaboration app for UI designers & frontend developers
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.
PSD to Sketch Design Converter - Convert a Photoshop design to a layered Sketch file for free
Craftwork - A collection of User Interface resources made by Craftwork