The Omni Group has brought OmniOutliner to the iPad. The app is more of a collection of functions that allow you to gather, categorize and later reference a variety of information. In fact, you can use OmniOutliner for spreadsheets, documents and even photo management. Putting all of that power on the ultra-powerful iPad will appeal to many people.
OmniOutliner can be as simple or as involved as you like. Start with simple lists. Move on to hierarchical documents and tables. The iPad version offers the thoughtful design you'd expect from an app