UI Design System - Starter kit for Sketch to create awesome user interfaces.
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.
Design Systems Repo - A collection of design system examples and resources
Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
Symbol Design System - Advanced web-design system based on Sketch nested symbols.
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project