After doing much research, downloading and testing of freeware and open source web editing software I stumbled across Flyspeck. It is hands-down the best of the lot. It allows my clients to edit their own sites but allows me to control what they can and can't edit. It's highly customizable - based on FCKeditor - and user friendly. The site could benefit from a support bulletin-board, but the documentation is solid. Customer support was friendly and responsive.