OmniGraffle
($49.99) Create website diagrams or process flows on your iPad. Mixed reviews from UX experts. Interaction designer Joshua Seiden thinks there are too many popovers, and interactions need to be based on two-handed multi-touch direct manipulation.
Adobe Ideas
(Free) Sketch design concepts, or capture and store color themes from photos on your iPad.
iMockups for iPad
($9.99) Quite good for simple mockups with basic user interface elements. Not appropriate for more complex wireframes, but good to whip up a new navbar or basic page layout.
