GNU TeXmacs is a free wysiwyw (what you see is what you want) editing platform with special features for scientists. The software aims to provide a unified and user friendly framework for editing structured documents with different types of content: text, mathematics, graphics, interactive content. TeXmacs can also be used as an interface to many external systems for computer algebra, numerical analysis, and statistics. New presentation styles can be written by the user and new features can be added to the editor using Scheme.
LyX is a document processor that encourages an approach to writing based on the structure of your documents, not their appearance. It is intended for people people who write and want their writing to look great without tinkering with formatting details, font attributes, or page boundaries. On screen, it looks like any word processor, but it uses the TeX engine for printed output and producing richly cross-referenced PDFs. It is stable and fully featured.
The ManEdit UNIX Manual Page Editor is an editor specifically tailored for UNIX manual pages. It has a preview viewer, uses the manual page XML format for easy editing, and comes with a tutorial and reference guide. It uses the GTK+ widget set and features syntax highlighting, a complete drag and drop system for easy viewing and editing, a crash recovery system, and sample manual page templates.
Turner will turn your boring old source code (C, C++, or Java) into colorized HTML ready to be displayed on a web page. Along with colorizing the source code, turner will also bolden keywords and italicize comments, convert tabs to spaces and optionally insert assorted usefull bits of HTML stuff (a 'home' link or a 'last modified' entry). While it's not the first of it's kind, nor the most versatile, it is small, fast, and totally self contained.