looks promising:
from http://insignificantthoughts.com/
Quote
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)
from http://insignificantthoughts.com/