I have to give my recommendations on iOS apps because there is just so much junk out there, its only necessary that I help you with some of my favorites.
SoftSign: Digitally sign documents with your iPad. This app syncs with Evernote and Dropbox. iPhone | iPad
OmniGraffle: The leader in Mockups and Flow charts iPad
App Cooker: A great mockup app for iPad. So easy to use. iPad
OmniFocus: The best GTD app out there. iPhone | iPad
Notability: Take notes really fast and easy for the iPad. iPad
Guest'd: If you plan on going to a trade show or event, this is great for people to use your iPad and to sign up to be on your mailing list. iPad
GoodReader: This app keeps PDF's organized and gives you quick access to them. iPad