There several spell checkers available, including those on Mail clients like Eudora or Outlook. But using them requires copying and pasting and many times we do not use them.
Other options as the one recommended by Micron is fine, but only works with MS Expolorer, at least for the moment. I use Firefox (a Mozilla based browser).
I still have to check the option by Zevang, I visited that page but have to spend more time understanding how it works, whether it's free and if it works with Firefox.
An embedded on-line spell checker, is obviously the best solution as it is independent of browser type an normally is very handy. In the case of digg.com, it appears as button besides the standard "submit" and "preview" buttons. In fact, it could replace the "preview" were it necessary.