Are you working on an iPhone App interface and workflow? Android? iPad? OSX? Or Facebook App? Then Keynotopia might be a good resource for you. Keynotopia Templates make it possible to design and prototype apps in minutes instead of hours. And all that in Keynote. Simply copy and paste interface elements from a Keynotopia template onto your slides, edit their labels, then add hyperlinks to make the interface clickable.
While it would never completely substitute the thorough UI design process in Photoshop for me, it would help me get a sense of what I am envisioning would work. Keynotopia.com
(thank you Bradley)