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/.
Anima App - Design, get feedback, convert to code, publish, iterate.
Zeplin - Collaboration app for UI designers & frontend developers
Sketch Repo - Collection of resources for anyone who uses Sketch
PSD to Sketch Design Converter - Convert a Photoshop design to a layered Sketch file for free
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.