User Interface stencils
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.
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.
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