This is a cool one, at least for iPhone app designers. Prototypes is a Mac app that lets you take those handcrafted mockups of iPhone screens and turn them into a clickable demo you can share with any iPhone or iPod touch user.
Prototypes takes just about any image format, including straight Photoshop PSD files. Once you drag them into your project, adding hotspots is as easy as clicking and dragging. Create a hotspot and then drag the link to whatever page should be loaded when it's tapped. You can assign a "back" action as well. All links can have a transition (in any direction) assigned to them. You can even add notes that display when a page is loaded and then disappear.
Aside from its ease-of-use, the real beauty of Prototypes is the free web service that comes with it. When you share a mockup from within the app, you get a ptyp.es url and a PIN. Have your client, your teammates, your boss or whomever log in to ptyp.es. They'll be asked to install the web app on their home page, after which they can enter your PIN and click through your mockup.
Prototypes currently only works on iPhones and iPod touches, but the developer expects to roll out iPad support in the next couple of months. At US$39.99, it's not a cheap investment, but if you deal with iPhone mockups regularly, this could be a lifesaver. The fact that you get the web service without a subscription makes the up-front price seem quite reasonable, at least to me. You can try out a pre-built prototype at the Prototypes website, and purchase it directly on the Mac App Store.