User Interface stencils
Free
Web Flowchart
Charles Parcell has created a very slick set of stencils for website flowcharting that has just about every component you might use for a site, from Flash to PHP to .Net. And all of them in a unique, colored style. Awesome.
Free
Lines
Just a variety of line styles and colors, ready-made. Customize this with your favorite line-styles.
Free
UX Activities and Documents
A stencil of UX activities and documents to illustrate the work that designers and researchers do. [originally created by Todd Zazelenchuk (Plantronics) and Elizabeth Boling (Indiana Univ)]
Free
Windows GUI Design
User interface elements for the Windows operating system, in case you have to design an interface where Aqua widgets are considered a strange and mysterious concept, and licking them is totally out of the question.