Ruby on Rails comes by default with the Prototype and Script.aculo.us Javascript Frameworks enabled to be used in AJAX calls and visual effects.

I found this nice article which proposes a ready-to-use debug popup window. All you have to do is add a simple command at the end of your application layout and a popup will show all the information regarding the request, session, response, flash variables and more.

Quick way to debug your web application.

Cheers!