Flat design style is on the rise and with it comes the need for flat UI elements for developers and designers to use. Designmodo has created an excellent Flat UI Kit for developers and designers to use in their projects. Flat UI is made on the basis of Twitter Bootstrap and includes both a PSD and a HTML version.
The Flat UI comes in both a free and paid version, the pro version starts at $39. The free version comes with buttons, inputs, button groups, selects, checkboxes and radio-buttons, tags, menus, progress bars and sliders, navigation elements and more. You can check out the demo and download the free Flat UI Kit. If you like the free version I do recommend checking out the paid version.