The answer to your question = Cushy CMS

I stumbled upon it the other day in my usual perusing of web blogs/forums.

All you need to do is know how to use classes in your site to define your editable regions. It is not exactly what I would consider full featured but if you're wanting your client to be able to edit content without messing up the site template than it's just what your looking for.

And yes, it's Free.