Aptana RadRails is an open source IDE for Ruby and the Ruby on Rails framework
featuring everything you need to get started fast and get even faster with
developing, testing, tuning, and deploying your Ruby and Rails apps.

Highlights include the following features:

 * Code Assist on Ruby and RHTML files
 * Outliner for code structure
 * Debugging support
 * Integrated server views
 * Dynamic testing
 * Integrated RoR deployment options

WWW: http://www.aptana.com/rails