Whether you're in school or running a business, being able to scan documents on the go can be extremely useful.
Notability: A flexible app that lets you take handwritten or typed notes, as well as read and annotate images and PDFs. You can record audio notes that are “synced” with your typed or hand-written ones (it lets you listen to the part of the meeting or lecture that corresponded to the time you took down particular lines or phrases) It also exports its contents to, among other options, Dropbox and iTunes. The iTunes export is especially useful, since you can just pluck your annotated files off iTunes when you plug in the iPad.