I just stumbled upon Aptana and I got to say I'm impressed. Its also open source, well depending on their definition of open source, either way the source code is not yet available but download the trial. Oh, it supports windows, mac and linux.

   Aptana is a robust, JavaScript-focused IDE for building dynamic web applications

  • 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