Microsoft Word (alas) or one of its generally even worse competitors. For business documents that need sharing with other people who have to use Microsoft Word so they can share them with other people who, etc.
TeXmacs, a more-or-less-WYSIWYG document editor with approximately TeX-compatible formulae and suchlike. I actually use this much more often than TeX or LaTeX.
Plain text :-).
HTML (either edited by hand -- horrible but in some contexts less horrible than (La)TeX -- or using a WYSIWYGish tool like BlueGriffon).
Someone mentioned Markdown. I've somehow managed to avoid ever using Markdown (not counting things like Stack Overflow and occasional blog comments) but it certainly belongs in the list.
