here is a popular, open source cms:
http://www.concrete5.org/
if you want something extremely simple and are willing to pay 25 bucks a month, you can use something like
http://www.surrealcms.com
Its basically a ckeditor type hosted cms. you build your site, add div tags with class=editable for every editable region on any page you want and you can set up a subdomain where clients can login, choose their page and edit away. You can add multiple editing sections on one page and when the client goes to edit that page, there will be tabs up top you can name for each editable section.
You can charge people a very low monthly fee and have this paid monthly after a few clients. If you setup a domain or subdomain you can customize the admin with your logo, colors, etc.
Best of luck to ya.