Flat UI is free to use retina ready UI kit brought to you by Designmodo. It has two versions, one layered PSD file and other HTML version. It is built on the basis of Twitter Bootstrap in a stunning flat-style.
Flat UI Free contains many basic and complex components which are great for designers to have at hand. The components includes buttons, inputs, button groups, selects, checkboxes and radio-buttons, tags, menus, progress bars and sliders, navigation elements and more.