Arcadia

Arcadia is a Light Integrated Development Environment (IDE) for Ruby language written in Ruby using the classic tcl/tk GUI toolkit. Some of Arcadia ruby IDE project features are: Script editing, running and debugging support. Source browsing, syntax highlighting, code completion. Working on any platform where ruby and tcl-tk work. Extensible architecture.

RDE(Ruby Development Environment)

RDE is a Ruby IDE and editor on Windows. RDE is more light and quick than other IDEs. RDE is the GUI wrapper of debug.rb and editor. You can edit, run and debug your scripts by GUI interface.
Features:

Use RDE like a favorite Editor.
Do execute ruby scripts from editor without saving files and get the result without dos console. 
Write scripts easily with HTML Help link, Input support features.
Edit the editing text with ruby on RDE.
Debug the scripts on RDE.
Platform: Windows ; License:no
Reference:https://homepage2.nifty.com/sakazuki/rde_en/index.html

Aptana RadRails 2

Aptana RadRails 2 is a free, professional integrated development environment (IDE) for Ruby and Ruby on Rails applications. It include support for project navigation, smart code assistance, visual debugging and structure outlining.

Platform:unknown ; License:unknown
Reference:https://www.aptana.com/products/radrails

FreeRIDE

A cross-platform IDE for the Ruby programming language. It is distributed under the same terms as Ruby itself.

Platform:Cross-platform;License:Ruby License
Reference:https://freeride.rubyforge.org/wiki/wiki.pl