Recently, I was writing a proposal for Intranet project. The proposal require do to browser based spell check dictionaries.
First I thought of use http://www.ie7pro.com/ plug-in for IE browser and http://spellbound.sourceforge.net/ for Firefox. both product fit and works fine for the requirement but the main issue is deployment and very difficult manage 2000+ users capacity. I am sure that solution this rise helpdesk call high and dose not suit for long term perspective.
While I was searching though found that RadSpell from <http://www.telerik.com/> standalone component which can be attached to any editable control, like textarea, div, iframe, or WYSIWYG editor (RadEditor). All you have to do is place the RadSpell tag on your page and set its ControlToCheck property to the ID of your editable element its also support the dictionaries of Microsoft® Word XP and 2003 (Word must be installed on the server). This gives end-users even greater convenience due to the richer set of dictionaries.
My proposal also required to add custom dictionary which RadSpell is enabled, the end-user can place unknown words to a custom dictionary, which will be automatically created for the respective language.