Material UI - A CSS Framework and a Set of React Components that Implement Google's Material Design
Prototyping Tools - Find the Prototyping Tool You Need
Bootstrap - Simple and flexible HTML, CSS, and JS for popular UI components and interactions
iOS Design Kit - The newest library of native iOS templates
Dropr - A really easy online portfolio for creatives.
Pencil Project - Single-user mockup / wireframing / diagramming tool