If you want an app to allow complete page creation and site design from scratch, there are lots of great apps available. One of my favorites is WebEdit Pro from www.interspire.com. It's relatively pricey, but very robust and professional.

If instead, you have templates or simple sites that you create for your clients, and then you want to give them the ability to edit pre-defined areas of the pages including images or even page content that you store in a databse, I developed an app named SnippetEdit. You can check it out at www.snippetedit.com.

The previous poster mentioned FCKeditor. I plan to incorporate FCKeditor into SnippetEdit instead of the old HTMLarea I use now. Main thing is that FCKeditor is compatible with Firefox as well as IE. FCKeditor is not really a standalone product. Rather it is an excellent component you can use to build a page/site editor.