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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.