Aptana - The Web IDE

Aptana is a beautiful web IDE based on Eclipse's RCP and has the following features:

• Code Assist on JavaScript, HTML, and CSS languages, including your own JavaScript functions
• Outliner that gives a snapshot view of your JavaScript, HTML, and CSS code structure
• NEW: FTP/SFTP uploading, downloading and synchronization
• Error and warning notification for your code
• Support for Aptana UI customization and extensions
• Cross-platform support
• Free and open source licensed under the Eclipse Public License, v1.0.
  

you can check it out at http://aptana.com

And the code assist is very different from the WTP's default Javascript code assist, it tells you what browsers support the tag or function that's in the completion popup.

JavaScript code assist

It has got a pleasant coloring for the HTML and JavaScript content.

HTML code assist

CSS code assist popup

I think combination of aptana and a web development IDE like RadRails or the WTP would be a powerful web development environment. Particularly for Ajax based modules.