WYSIWYG</b>
<input type="button" name="wyson" value="On" onclick="javascript:tinyMCE.execCommand('mceAddControl',false,'content');" />   
<input type="button" name="wysoff" value="Off" onclick="javascript:tinyMCE.execCommand('mceRemoveControl',false,'content');" />
<br>

Advantages: After to switch OFF you can use Quicktags as usually.
You can add --more-- and --nextpage-- tags as well as edit HTML directly.
Next you can switch back to WYSIWYG and complete your work.

There is a picture how it looks: http://wordpress.site.cz/images/wpadmin.gif

++++++++++++++++++++++++++++++++++++++++++++++++

Here is ColdForged's original code to turn SpellingChecker on or off:

++++++++++++++++++++++++++++++++++++++++++++++++

Here is how I combined the two (by default I have wysiwyg turned on):

1). Add this to the top of the page:

2). Then delete the Speller buttons and change the WYSIWYG on/off buttons to include
the SpellingChecker commands:

++++++++++++++++++++++++++++++++++++++++++++++++

Now if I can only figure out how to do the same thing with Quicktags!

++++++++++++++++++++++++++++++++++++++++++++++++

Update:

OK, finally figured out how to turn Quicktags on or off also! Now when I start in default
wysiwyg mode (edit_form_advanced.php), SpellingChecker is turned off, and there are no Quicktags!
When I turn wysiwyg off, then the SpellingChecker button appears (and works), and also the
Quicktag buttons appear (and work). This is done through javascript, and switching css styles.
And there is just one set of buttons to switch everything on or off.

Hope someone finds this useful.... here's what I did:

1). Add swapstyle.js to wp-admin folder (code at bottom of page)

2). In wp-admin.css change the quicktags style to this and save the file:

#quicktags {
	display: none;
	margin-right: 1%;
}

3). Now change the style to this and save the file as wp-admin-showqt.css:

#quicktags {
	display: inline;
	margin-right: 1%;
}

*** You now have two nearly identical css files in the admin area...
just the quicktags display property is changed.***

4). Add the following to top of edit_form_advanced.php (change file names and titles as you want):

*** now the top of my edit_form_advanced.php page looks like this: ***

<body onload="showHideSpeller(0)">
<script type="text/javascript">
        function showHideSpeller(showIt) {
                var button = document.getElementById("spellingdiv");
                if( showIt ) {
                        button.style.display = "inline";
                } else {
                        button.style.display = "none";
                }
        }
</script>
<script src="swapstyle.js" type="text/javascript" language="javascript1.2"></script>
<link rel="stylesheet" type="text/css" href="wp-admin.css" title="hidequicktags" />
<link rel="alternate stylesheet" type="text/css" href="wp-admin-showqt.css" title="showquicktags" />

5). Put the following in edit_form_advanced.php where you want the quicktags to show
(I added the div tags to enable css switching):

6). Change your on and off buttons to this:

7). I found the swapstyle.js at http://www.howtocreate.co.uk/jslibs/htmlhigh/swapstyle.html
here it is:

/***********************************************************************************************
                             Script to swap between stylesheets
    Written by Mark Wilton-Jones, 05/12/2002. v2.2 updated 29/04/2004 for Konqeror XHTML fix
    
    Please see http://www.howtocreate.co.uk/jslibs/ for details and a demo of this script
Please see http://www.howtocreate.co.uk/jslibs/termsOfUse.html for terms of use
************************************************************************************************