I've been working for awhile on Rails plugin that makes it easy for users to edit their own content. I started it to scratch an itch I've had for a long time. The problem I have with most CMS systems is that they want run your whole site. Sometimes this is ok, but a lot of times you have a rails app where you have pieces of it that it would be nice to to let your client edit for example. For those cases, the edit_me plugin is your friend.
The idea is pretty simple: when in editing mode, edit_me gives you an edit icon next to the content produced by rhtml and html.erb files (you can configure which files you want to be editable). Click the edit icon and up pops an editor (using wymeditor). Save your changes and the page reloads with your changes. Edit me assumes you're using git (why wouldn't you?) and lets your roll back your changes in the History tab of the editor.
To check it out, head on over to http://github.com/superchris/edit_me/tree/master and follow the instructions in the README.