Aptana - Javascript focused 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. (Source available soon)