Scrivener is a powerful content-generation tool for writers that allows you to concentrate on composing and structuring long and difficult documents. While it gives you complete control of the formatting, its focus is on helping you get to the end of that awkward first draft.
SmartEdit is an add-on that can be applied to Microsoft Word, adding features that mostly fall within the school of ‘Advanced Editing’. For a start, it flags repeated adverbs and phrases, keeping a count to alert author of overuse or problematic repetition. It also enhances Word’s error-checking facilities to pick up on inconsistent, rather than just incorrect, punctuation and grammar.
Free software! Available for Linux, Windows and macOS, FocusWriter is designed to eliminate distractions so you can actually get on with the job of writing. To that effect it enables you to hide other apps, customize the way your text appears on screen and keep track of your progress. It blocks all distractions, has timers and alarms and an auto-save function. If you're feeling particularly old-school you can even add typewriter sound effects.
LibreOffice is a free, open source alternative to Microsoft Office, and that means its word processing app, Writer, has many of the power features of Word without the price tag. It has excellent format compatibility, document template galleries and support for plugins. One of its most useful features is the LibreOffice Template Center, where you'll find templates for all kinds of content.
This is a free, highly-rated desktop publishing application for Linux, OS X and Windows that's suitable for producing entire magazines. Around since 2001 and regularly updated. Although it's a little tricky to use at first, it offers professional-grade publishing with layered, multi-page documents and good color management support. If you've ever used Adobe InDesign, you'll find them very similar.