I have been working with Ruby (On Rails) those days together with NetBeans, Oracle dropped the support for Ruby when NetBeans 7.0 was released but do not worry!, the community has developed a plugin to add support for Ruby on NetBeans again, just follow the steps:
1. Download this plugin and unzip it.
2. In NetBeans, Tools -> Plugins -> Downloaded -> Add and select all the files (including the JAR otherwise it will not work). Be careful on this step, you need to change the default type when selecting them from default to all so you are able to pick the JAR too.
3. Follow the installation process.
4. Now you have support for Ruby on NetBeans 7.3.1!