The JRuby AST Viewer
If you build the contrib/gsf.tools (ruby/gsf/tools for CVS) module, your Windows menu will contain some extra items helpful to developing Ruby+NetBeans support:
In particular, the AST viewer lets you peek at the AST generated for the current editor buffer. As your click on items in the source editor, the AST viewer warps to expose the corresponding AST node and its surrounding hierarchy:
Conversely, if you click on AST nodes, the corresponding editor regions are highlighted in purple:
You'll find further building instructions in the RubyBuildInstructions document.