Git Sketch Plugin - Version control for designers
Whimsical Wireframes - Lightning fast wireframing with real-time collaboration.
Fluid for Sketch - Sketch plugin to make your responsive designs more efficient
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
Sketch Plugin Course by Design+Code - Enhance your design workflow by creating a Sketch plugin
Invision - Prototyping and collaboration for design teams