User Interface stencils
Use this stencil to represent words, ideas, and tasks arranged around a central idea.
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.
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.
Much interface design at Omni has started with a quick mockup in OmniGraffle. Here's the stencil I've been building, adding elements as I need them. Most designs start with an existing screenshot, with swatches of controlColor over things we need to get rid of, and layers of new controls on top of that. Clearly there are lots...more