I've got a client who wants to integrate a CMS into an existing Rails application. As a result, I've started in on an evaluation of what's out there, to help us make the buy-vs-build decision. Here's the list of potential software that I came up with:

Rails CMS alternatives

Active projects:

adva-cms

• repo: http://github.com/svenfuchs/adva_cms/
• site: http://adva-cms.org/
• Last update: 11/24/09
• "the cutting edge Rails CMS platform"
• Engine-based CMS

BrowserCMS

• repo: http://github.com/browsermedia/browsercms
• site: http://www.browsercms.org/
• Last update: 11/25/09
• "Humane Content Management for Rails"
• gem-based dedicated CMS

Goldberg

• site: http://groups.google.com/group/rubyonrails-goldberg
• repo: http://github.com/urbanus/goldberg (rails 2.3 compatible fork at http://github.com/lardawge/goldberg)
• "the rapid website framework for Ruby on Rails"
• Last update: 4/28/09
• plugin-based CMS

Kete

• repo: http://github.com/kete/kete/
• site: http://kete.net.nz/
• Last update: 11/20/09
• "a mashup between content management and knowledge management"
• Dedicated app, closer to wiki/blog than CMS

Radiant

• repo: http://github.com/radiant/radiant
• site: http://radiantcms.org/
• demo: http://radiantcms.org/demo/ (admin/radiant)
• "Radiant is a no-fluff, open source content management system designed for small teams."
• Last update: 11/13/09
• Engine-based CMS, mature project

Refinery

• repo: http://refinerycms.org/
• site: http://refinerycms.com/
• demo: http://demo.refinerycms.com/session/new (demo/demo)
• "a simple, easy to use, well presented content manager for small businesses."
• Last update: 11/24/09
• Dedicated CMS built on top of Rails

Seed

• repo: http://github.com/desaperados/seed
• demo: http://seed.mediacollectiveslice.com/ (admin/administrator)
• Last update: 9/22/09
• "a Common Sense CMS"
• Dedicated CMS built on top of Rails

Static

• repo: http://github.com/trevorturk/static/
• site: http://almosteffortless.com/static/
• demo: http://static.heroku.com/
• "super simple Rails CMS built for Heroku"
• Last update: 10/16/09
• Dedicated CMS built on top of Rails

Station

• repo: http://github.com/atd/station
• site: http://rstation.wordpress.com/
• "Station is to Rails in Content Management System like Rails is to Ruby in Web Development."
• Last update: 11/21/09
• Engine-based CMS

Typus

• repo: http://intraducibles.com/projects/typus
• site: http://github.com/fesplugas/typus
• demo: http://typus.heroku.com/ (user@intraducibles.com / columbia)
• "Trusted users editing structured content"
• Last update: 11/20/09
• Not really a CMS, more of a way to spin up an administrative backend via generators

Zena

• repo: http://github.com/zena/zena
• site: http://zenadmin.org/en
• "CMS (content managment system) with a focus on usability, ease of customization and web 2.0 goodness"
• Last update: 11/25/09
• Dedicated CMS built on top of Rails

Inactive projects:

Ansuz CMS

• repo: http://github.com/knewter/ansuz
• site: http://ansuzcms.com/
• "an open source Content Management System built in Ruby on Rails."
• Last update: 3/19/09
• Dedicated CMS built on top of Rails

Comatose

• repo: http://github.com/darthapo/comatose/
• site: http://comatose.rubyforge.org/
• "a micro CMS implemented as a Rails plugin"
• Last update: 2/26/09
• plugin CMS functionality for any Rails app

Geego

• repo: None - versions available by download
• site: http://www.geegocms.com/
• "Manage content with style"
• Last update: 2007?
• Dedicated CMS built on top of Rails

Thetis

• repo: None - versions available by download
• site: http://thetis-sysph.sourceforge.net/
• demo: http://sysphonic.com/thetis/
• "The World-Strongest Groupware"
• Last update: Jan. 2009?
• Dedicated CMS and groupware app built on top of Rails

Dead projects:

Rubricks

• repo: http://rubyforge.org/projects/rubricks/
• site: http://rubricks.org/index_en.html
• "Ajax style content management system powered by Rails"
• Last Update: Nov. 2008

Widgetfinger

• site: http://widgetfinger.com
• Thoughtbot site that allowed generating brochureware right in the browser.
• Scheduled to go offline 12/21/2009

Related

Blog engines

• Enki
• Mephisto
• Typo

Static page generators

• Jekyll
• Nanoc
• StaticMatic
• Webby

Wikis

• Redmine (also project management)
• Wagn

I've also put this list online as a gist. I'll keep that version up to date with corrections and additions.