LyX is a document processor that encourages an approach to writing based on the structure of your documents (WYSIWYM) and not simply their appearance (WYSIWYG). LyX combines the power and flexibility of TeX/LaTeX with the ease of use of a graphical interface.
LyX is released under a Free Software/Open Source license, runs on Linux/Unix, Windows, and Mac OS X, and is available in several languages.