Here’s an interesting open source tool: Nvu – The Complete Web Authoring System for Linux, Macintosh and Windows Nvu allows page editing using tools based on the Mozilla composer and FTP. It claims to work on any platform and looks promising, although I have not downloaded and tried it myself.
While we’re talking open source, let me not forget Plone, which is an open source content management system with plenty of support and help. Plone works on any platform and is based on Zope, but you don’t have to know Zope to use it.
ADDENDUM May 12, 2011: The developers of Nvu have released a new open source WYSIWYG editor called BlueGriffon that you should look at if you’re looking for free software for web design.