I use RadRails, and yes, you can add other cool Eclipse plugins. But to add a cool plugin, you have to add not only the plugin itself but all plugins on which it depends. I've successfully added WickedShell to RadRails, but to do this I also had to swipe org.eclipse.pde and org.eclipse.jdt from a standard Eclipse installation, and add these plugins to the RadRails plugins directory.
···
RadRails (www.radrails.org) if you plain on doing any Rails
programming.
RadRails is based on Eclipse, RDT and custom develpment.
(doesn't have HTML code complete, code complete from classes are hit
and miss,
doesn't really allow you to add other cool Eclipse plugins)