I have been using a free spell checker for my Dirt Doctor forum posts for about two weeks now. I really like it. It is simple to use (right click on your message and select Check Spelling), you can add new words to the dictionary, and it is FREE. I don't get any advertising with it, either. So far I am really happy. Here's a link to the source.

http://www.iespell.com/

It only works on Microsoft's Internet Explorer, so I guess you should know about that.