Aptana RadRails
Free, open source IDE for Ruby and Rails applications
RadRails is now included as part of Aptana Studio 3. No extra download required--now integrated into a single editor.
Core Capabilities

Code Assist
Quick access to Ruby, HTML, CSS and JavaScript structures, methods, elements, properties, tags & attributes. Helpful information about the element also shown. Supports your custom code as well.
“I'm really starting to like RadRails, especially as a plugin since it's nice to be able to use one IDE for everything I do.”
@havoc_joe, University of Maine
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