At 5k stars, rebass is a “React primitive UI components built with styled-system”. It’s lightweight, and beautifully designed set of 8 extendable, consistent and reusable components built with responsivity and customizability in mind (with built-in themes). Here are the components.
Atlassian’s React UI component library, implementing Atlassian’s design guidelines and putting styled-components to use. The library was started 2 years ago as an effort to implement ADG3 designs for all Atlassian Cloud products, which are built on top of React. Every component is a package.