OmniOutliner Pro
One of the primary tasks of an Information Architect is to organise information so people can easily find what they need. Out of this organisation, a navigational structure is created. The tool I most often use for this task is OmniOutlner Pro, from OmniGroup. The application takes seconds to learn basic tasks, such as adding items, promoting and demoting items, and adding notes to items.
However, if you take the time and read the (quite accessible) manual, you discover all sorts of time saving ideas and advanced features including voice recording, global styles, and very good integration with OmniGraffle, and AppleScript. My favourite feature of OmniOutliner Pro is the ability to take an outline and open it directly from OmniGraffle Pro, to create an instant “one click” sitemap. That feature alone gives it a place in my toolbox.