Isaac Williams via sayaka maruyama onto UI For Apps: Large Collection Of Inspiring App Designs! (User Interfaces, collection by:

Free Awesome Flat Ui Kit

Really great Flat UI kit we at C&C found: Free Awesome Flat UI Kit A kit showing a flat UI design.

RED UI – Another free UI kit

RED UI is another free (but awesome) UI kit from CSSauthor. The PSD includes several elements like dropdown menus, forms, progress bars, etc.