If you use Adobe Fireworks for creating mockups (as I do), and have any need to create mockups for iPhone apps, you need this free Fireworks toolkit of iPhone UI elements from the folks who created Notespark.
It’s a fairly comprehensive set of UI elements, and everything is drawn as a vector object. I had started to create something like this for myself, but didn’t get very far — now I don’t need to finish the work.
