Try As-U-Type (http://www.asutype.com). It is a global instant spell checker that works with all applications, including web browsers, forums and all IM apps. The best of it is that it can correct your typos immediately and automatically as you type so you don’t need to stop typing and go back to fix the mistake yourself. Shareware, free 30-day trial, costs 30 bucks, worth have a look.
–jd