SCREEM is a GNOME website / tag based htmleditor (ie not WYSIWYG) which aims not only to aid in creating web pages, but also to provide useful site maintainance facilities as well, including automatic link updating, and site upload facilities.
The CMS-Bandits is a set of php scripts, with online htmleditor, calendar, search engine, rss reader, revision log, personal nickpage, comment system, webcrawler and even more.
Pandereta(little tambourine) is a C# based WYSIWYG editor for HTML pages. It's simple but powerfull too, and allow to extends it by the use of plugins. Uses gecko engine to preview pages.
Kafenio is a Wysiwyg HTMLEditor for Browsers that support Java 1.3 and newer. Kafenio grew out of Howard Kistlers Ekit Editor. Main objectives: checkstyle compliant code, reusable in any java application, higher stability and improved usability.
A simple Unicode text editor which supports many encodings. Also nice Notepad replacement.
To be a simple to use and fast Unicode HTML/XML editor. I have the intention to make it the Gnome2 editor.
Yet no syntax highlighting.
Peacock is a HTMLEditor for GTK+/GNOME. It supports most of basic HTML. It features Session Management and HTML Preview using the GtkHTML widget, Drag N' Drop among other things. Its distributed under GPL.