For just writing a text document, use a text editor. I like TextMate, but emacs, vim, BBEdit, or notepad works just fine. Use Markdown for basic formatting. (This includes preparing manuscripts that someone else will be responsible for typesetting)
For math (or other formula-heavy) papers, use LaTeX.
For typesetting anything else that needs to look pretty printed (e.g. resumes, brochures, technical documents, posters, books), InDesign is so so so much better than Word.