In Netbeans 7, Ruby support was dropped:
Although our Ruby support has historically been well received, based on existing low usage trends we are
I followed the steps described in this blog post - http://blog.enebo.com/2011/02/installing-ruby-support-in-netbeans-70.html and it works. Enjoy
Click Tools -> Plugins Click on
'Settings' tab Click on 'Add' button
to get Update Center Customizer popup
Set name to 'Beta 1' Set URL: to 'http://updates.netbeans.org/netbeans/updates/7.0/uc/beta/stable/catalog.xml.gz'
Press 'OK' Click to 'Available
Plugins' Click 'Reload Catalog' Choose
'Ruby and Rails' Pat yourself on the back
Edit: now ruby on rails plugin can be found directly at "Tools">"Plugins">"Available Plugins">"Ruby And Rails" (if you don't find this plugin at the provided path you should download and install the latest netbeans ide)
Reedit: if you need Ruby On Rails support for Netbeans 7.1 check http://blog.enebo.com/2012/01/workaround-for-ruby-support-on-netbeans.html