10 Free Flat UI Kits http://webdesignledger.com/freebies/10-super-useful-free-flat-ui-kits
Flat UI. Free Web User Interface Kit
Material Design - Style - Color-UI
Flat UI Colors
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.
I like the way that they try and bleed the screen into the Phone, the phone UI always try's to define its boundaries, maybe it shouldn't?