I've collected all open source CMS based on rails 3.x.
Rails CMS/Wiki/Forum
This is a very simple but feature rich CMS framework. It is good startpoint for a new rails project what will be wiki, Forum or CMS
Source: https://github.com/jlapier/Rails-CMS-Wiki-Forum
Casein CMS
This is a very minimal and lightweight CMS. Only users table created on rak db:migrate. It is a good start point for any content based application. This application scaffolding your rails app with a very beauty user interface.
Website: http://www.caseincms.com/
Source: https://github.com/spoiledmilk/casein3
ADVA CMS2
This CMS is not running on ruby 1.9.2. So I can't try out it.
Website: http://adva-cms.org/
Source: https://github.com/svenfuchs/adva-cms2
Refinery CMS
This CMS is work perfectly out of the box. It useable by a non programmer user too.
Website: http://refinerycms.com/
Source: https://github.com/resolve/refinerycms
Demo: http://demo.refinerycms.com/refinery
Radiant CMS
This is a very old CMS with still active development team. I think this is a usable CMS for a programmer.
Website: http://radiantcms.org/
Source: https://github.com/radiant/radiant/tree/rails3
Demo: http://radiantcms.org/demo/
Typo weblog engine
This is a blog engine, but you can use as CMS.
Website: http://fdv.github.com/typo/
Source: https://github.com/fdv/typo/
Demo: http://demo.typosphere.org/
Update (2011-07-18):
Locomotive CMS
A great NoSQL CMS. It uses MongoDB. I'm very happy to see this new CMS.
Website: http://locomotivecms.com
Source: https://github.com/locomotivecms/engine
Demo: http://demo.typosphere.org/