To help you get started with flat design, we have compiled some free flat PSD Web UI kits. All of them are editable in PSD format and mostly covering all of the basic UI elements such as textfield, checkboxes, radio buttons, buttons, drop down menu and some other cover extensive list of elements.
White Stripes UI Kit – Free PSD
Free Flat UI Kit
Freebie: Flat User Interface Kit (Vector PSD)
Free PSD UI Kit
Flat UI kit (psd)
FLAT UI ELEMENTS
Free Ui Kit
Free PSD Web UI kit that includes a set of beautiful components, which can be used to create websites and applications.