In Netbeans 7, Ruby support was dropped:
Although our Ruby support has historically been well received, based on existing low usage trends we are
Ruby on Rails support is now included in the default modules list. I have not been able to find a download with just the netbeans ide (no additional plugins) so I download the php bundle and then uninstall all the php plugins and install the Ruby on Rails plugin.
I would suggest also installing the git and scss plugins. The scss plugin is not yet in the default plugin list for 7.0, but you can get an nbm for it here. I install the 0.1 version, not the 0.3 beta version.
The full list of plugins I REMOVE are: Bugzilla, CVS, Hudson, Mercurial, PHP, PHP Documentor Tag Help, PHP Symfony Framework, PHP Zend Framework, Software as a Service, Subversion. Of course if you use any of these, you may not want to remove them.