User Interface stencils
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.
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.
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.
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.