I've used this in the past, for class projects and all

http://www.cushycms.com/en

Basically, they require you to put a certain tag...[cushcmys] or somethign similar to the areas you wan't the client to be able to edit...This will only let them change that certain area without messing up the site. I belive so the trial works for a couple of sites that you can let your clients handle. But if you have big amount of clients, you can go for the PRO version. It will let you manage thousands of web-sites