Some years ago I created a plugin for FCKeditor to easily provide a preview of the contents that are being edited as they will look in the final site. That's specially important when you're using special markup in the editor that it's being replaced at the server by a widget or just some plain data.
The plugin is quite simple as the idea itself is simple: Get the current contents, and instead of opening a window with that HTML, post it to a custom page at the server and that page will be the one that can provide the proper context and show the HTML as it will be rendered in the final page.
This weekend I was tasked by Flatbooster GmbH to upgrade it so it can be used with CKEditor, and also perform some additions like controlling the size of the preview window.
The usage is quite basic, you just have to add it to your extraPlugins, specify the serverPreview_Url with your server script and use the posted "htmlData" variable. You can read the install.html to learn about other little options.
The plugin replaces the normal 'Preview' button, so there's no need to do anything to your toolbar.
The plugin can be downloaded here: Server Preview plugin for CKEditor 1.4.
Version 1.4, allow to use other CKEditor instances in the serverPreview_AdditionalFields setting.