Amaya is a complete online web browsing and authoring environment and comes equipped with a WYSIWYG style of interface, similar to that of the most popular commercial browsers. With such an interface, users do not need to know the HTML or CSS languages.
NVU (pronounced N-view, for a "new view") makes managing a web site a snap. Now anyone can create web pages and manage a website with no technical expertise or knowledge of HTML.
Quanta Plus is a web development tool for the GNU/Linux K Desktop Environment. Quanta is designed for quick web development and is rapidly becoming a mature editor with a number of great features and lies at the heart of the KDE Web Dev suite of tools.
Bluefish is a powerful editor for experienced web designers and programmers.
Bluefish supports many programming and markup languages, but it focuses on editing dynamic and interactive websites.
Mozilla's HTML editor keeps getting better with dynamic image and table resizing, quick insert and delete of table cells, improved CSS support, and support for positioned layers. For all your simple documents and website projects, Composer is all you need.
Screem is a web development environment. It's purpose is to increase productivity when constructing a site, by providing quick access to commonly used features. While it is written for use with the GNOME desktop environment in mind it does not specifically require you to be running it, just have the libraries installed.
Unlike most other HTML editors SCREEM does not provide a WYSIWYG display of pages. Instead you are presented with the raw html source in its editor window.
OOWeb is a WYSIWYG web site design tool, part of the OpenOffice suite. It has features very similar to that of Microsoft's FrontPage.
For Windows user I would recommend HTML Kit
HTML-Kit is a full-featured, highly customizable and free development environment that can be used to create, edit, validate, preview and publish web pages and scripts. Despite its name and the light download size, HTML-Kit is a multi-purpose tool that has support for several scripting and programming languages. Over 400 free plugins are available for extending and customizing HTML-Kit to fit the way you work.