It's part of the Google toolbar and it's perfect for discussion boards. You type your message into the box, then you click on Google spellcheck. It works instantly, it highlights all your spelling errors in red, and you can click on each one to get a list of suggested spellings to select from. You can ignore any highlighted errors that aren't really errors, like member names. You can find it at:
I think they have it for both IE and FireFox.
--Percy