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Whether used for simple note-taking or as a full-blown editor, Evernote is a great tool for writers. The ability to capture information of just about any type (text, voice, image) make this app a surprisingly capable helper for no cost.
Free (Premium Evernote service available at cost)
Writing means research, and Wikipedia is always a big part of that. This app helps leverage Wikipedia to the fullest extent possible on the iPad.
The search function is first-rate, and the article display outstanding. Articles can be queued up for later reference and saved for access offline.
Mind mapping is a great way to plan writing projects and this app is optimized for good use of the iPad interface.
Full graphical maps can be easily created, and rearranging topics as easy as dragging them on the screen.
The app can output outlines from maps for those who prefer an old-school approach.