Aptana: The Web IDE Cross-platform support – That’s right, you can use it on Windows, Mac or Linux. It’s not a gui, WYSIWYG editor like front page or dreamweaver, but it does have code assist and an instant preview so you can see what you’re doing as you code.

They have some awesome videos so you can see how it works even before you install it. Built in support for several popular javascript libraries (with code assist!) and CSS support. It even shows you which features are supported by the various browsers as you type.

Oh yeah, it’s FREE! Free to use and also open souce. You can download it as a standalone app or as a plugin for an existing installation of Eclipse 3.1. (3.2 partially supported)