Quommunnication Stencil Kit
A PSD file with common design elements for sketching and wireframing: form elements, RSS feed icons, colors, Advertising units, browser windows and grids

Facebook Applications Stencil Kit
A pretty sizable collection of Facebook related elements to use in creating wireframes for Facebook applications.

Free Sketching & Wireframing Kit
The Sketching & Wireframing Kit is a free set of elements for sketching and wireframing. It consists of form elements, icons, indicators, feedback messages, tool tips, navigation elements, image placeholders, embedded videos, sliders and common ad banners. The Kit comes in two vector formats, one for Adobe Illustrator and the other in SVG, which can be easily modified. It can also be downloaded in PDF and EPS formats.