It's been requested before to be implemented on this site, but all browsers currently support spell checking to allow you to post correct spelled messaged.
Just to help you guys out, here's a quick guide:
Firefox: https://addons.mozilla.org/en-US/firefox/browse/type:3
This enables spell checking in big text areas (the post content), but if you also want checking in the title fields, please check this page out: http://www.howtogeek.com/howto/uncatego ... ut-fields/
Opera: http://www.opera.com/browser/tutorials/spellcheck/
Internet Explorer: http://www.iespell.com/