Vertical Infinity is a huge flat-style UI kit containing a lot of web elements (menus, dropdown lists, buttons, form elements, checkboxes and radio buttons, toggle, navigation and paginations, tags, alert boxes, progress bars, login forms, contact forms , weather widgets, etc.) which you can find very useful. It is an awesome PSD pack released by CSSAuthor.
Download freebieVertical Infinity: Flat Style UI Kit PSD
PSD