Mellel: "word processor for Mac OS X designed especially for scholars, creative and technical writers" (Commercial, Mac)

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Open Office: "the leading open-source office software suite for word processing, spreadsheets, presentations, graphics, databases and more" (Free, cross-platform)

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Scrivener: integrated writing platform for novelists, scholars, and others who want to manage the writing process seamlessly (Commercial, Mac)

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TextExpander: Insert frequently used text with just a few keystrokes; avoid typing information over and over again. (Commercial, Mac; via Ryan Cordell, ProfHacker)

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WriteRoom: full-screen word processing environment promotes focus on the writing task (Commercial, Mac)