User Interface stencils
Free
Lines
Just a variety of line styles and colors, ready-made. Customize this with your favorite line-styles.
Free
Web Form Elements
Edward Anastas has put together a handy stencil composed of often-used elements in HTML and web page forms: radio buttons, checkboxes, combo boxes, entry fields and text areas. Nice looking and Aqua, to boot.
Free
UML 2
Additional UML diagram tools, esp. for Use Case and Activity diagrams. There is also a Domain object for class domain or namespace groupings in Class and Collaboration diagrams.
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.