I know one forum that has a spell check next to the submit button.  Sadly, few seem to ever use it.  Scanning some messages here, I saw a "there" where "their" was meant, and it made me cringe.  That's not something a spell-check would help with, but it does remind me of the feature I've only seen once.  If people will be commonly running a page through Babelfish to translate languages, proper spelling and grammar is more important!  (I should tell you about a baking lesson that said to very carefully hang your nephew from the ceiling with a rubber spatula.)  I've not installed a forum yet, but it would be good if it was easy for an administrator to copyedit posts.

There is/was a plugin that Matin did to add a spell check.... But have you seen a freebie http://www.iespell.com ?