Flat UI. Free Web User Interface Kit
Flat UI Colors
Material Design - Style - Color-UI
Flat UI Elements PSD
Flat UI Free - Flat UI Free is made on the basis of Twitter Bootstrap in a stunning flat-style, and the kit also includes a PSD version for designers.
10 Free Flat UI Kits http://webdesignledger.com/freebies/10-super-useful-free-flat-ui-kits
flat-UI-design-577x1024 Top Mobile App Design Trends 2015&2016, important visual elements for great UI