I came across cushy CMS lately. The premise is that you sign up for an account on the cushy cms website, supplying ftp details for the site you want to edit and then just add a class="cushycms" for every tag that is to be editable. You then set up editors who get an email with the details they need to log in. Supports the usual - input fields, text areas and images.

I guess the main concerns are that the editor would see the cushy cms branding (think you can pay to do your own branding), they might go out of business, and they might change the model so that the free stuff becomes pay-for.