LyX is an advanced open source document processor that encourages an approach to writing based on the structure of your documents, not their appearance.
LyX lets you concentrate on writing, leaving details of visual layout to the software.
LyX was originally a Unix application, but now runs natively on Windows and MacOS X as well, thanks largely to the cross-platform Qt toolkit.
LyX produces high quality, professional output -- using LaTeX, an industrial strength typesetting engine, in the background; LyX is far more than a front-end to LaTeX, however.
No knowledge of LaTeX is necessary to use LyX, although it will give a user more power.
LyX is stable and fully featured.
It has been used for documents as large as a thesis, or as small as a business letter.
Despite its simple GUI interface (available in many languages), it supports tables, figures, and hyperlinked cross-references, and has a best-of-breed math editor.
