Ruby on Rails
TextMate

TextMate (homepage) is a text editor for Mac OS X featuring featuring syntax highlighting, code autocompletion and much more. It’s fantastic for Ruby (and Rails).

There are several bundles recommended for TextMate

Bundle Description
RubyOnRails.tmbundle Must-have collection of code snippets and macros to make Rails development in TextMate more efficient.
RubyAMP.tmbundle TextMate bundle called a “Must Have” by many. Adds several features essential for any development environment (IE: Ruby-debug integration and enhanced code navigation).

See Also