I still haven't figured this out yet, and none of this is verified, but I'm putting together the information I've found on the forums, in hopes that someone else may correct the information and evenaulty have a nice how-to soon.

Installing and configuring aspell

1. Install the following libraries.

• aspell 
• aspell-devel
• aspell-en (or your language of choice)

2. Install Text::Aspell using cpan.

3. Modify configuration file to include reference to the dictionaries:

#cd /data/WebGUI/etc

#gedit www.mywebsite.com.conf

     "availableDictionaries" : [
       {
        "id" : "en",
        "name" : "English",
        "default" : "1"
      }
    ],

4. modify the 'Forum Rich Edit' Template from the WebGUI Asset manager (i.e. Root > Import Node > RichEdit > Forum Rich Edit ) to include 'Server Side Spell Checker' by clicking the appropriate check box in the properties.

5. add it to the content managers rich edit: assets->root ->import node-> richedit -> Content managers Rich edit and add Spellcheck.

another user commented that aspell doens't work with tinymce. I haven't verified this.

adding other languages to tinymce3

You can add other languages to tinymce. Available language packs can be found here: http://services.moxiecode.com/i18n/. On this page is explained how to contribute as well.

Currently version 3 (3.05) of TinyMCE is used in WebGUI 7.5.21. Their documentation can be found here: tinymce.moxiecode.com.

spellchecker for IE (only)

ieSpell v2.2.0 build 647 seems to support other languages (http://www.iespell.com/). Commercial use requires license ($$). To enable this in webgui:

1. check the box called "spellchecker IE only for the 'Forum Rich Edit' Template. This is found from the WebGUI Asset manager (i.e. Root > Import Node > RichEdit > Forum Rich Edit )
2. Similarly for the Content managers Rich editor.
3. Checkin your changes.

spellchecker for firefox

you can use ietab plugin

Other options

Xinha Here: WYSIWYG in browser: https://addons.mozilla.org/firefox/1449/

Says it works nicely with spellbound:
http://spellbound.sourceforge.net/

EDIT: Spellbound won't install on 1.5.0.3 Oops. Aspell is likely to work: https://addons.mozilla.org/firefox/1022/