Have a look at Aptana: The Web IDE. Aptana is a robust, JavaScript-focused IDE for building dynamic web applications. Highlights include 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
• Error and warning notification for your code
• Support for Aptana UI customization and extensions
• Cross-platform support
• Free and open source

Please notice that the Aptana code assist feature has support for different popular AJAX Javascript libraries such as Dojo and Prototype.

The alpha version was released in March 2006 and Aptana is now in beta with the current version being 0.31. You can either download Aptana as a stand alone client or as a plugin for Eclipse.

I’ve tried the stand alone client and I really like the code assist and the integrated JavaScript and DOM documentation.