Elemental-UI is a high quality, modular set of UI scaffolding components and controls for React that are built from the outset to natively implement React patterns.
Elemental-UI is inspired from many component based UI libraries and is like a enhancement to Bootstrap. it's a must try if you're a fan of that.
React & CSS UI Framework.
Currently under development; built by @JedWatson and @JossMac at Thinkmill.
See elemental-ui.com for examples and documentation.