If I understand you right, you are looking for an Eclipse-plugin?!

In this case you should give RadRails a try (<a
href="http://www.radrails.org/">RadRails-Website</a>). You get a
Rails-framework and a Ruby-IDE too. I recommend to use the integrated
updater in Eclipse (Help -> Software Updates -> Finf and Install).

As you are using MacOS, I hope you already came across <a
href="http://www.macromates.com/">TextMate</a>, the best editor on
earth and *the* killer-app of MacOS. It's a texteditor on steroids, and
in my oppinion far better than RadRails.

Hope that helps,
napoleon