If you are not using Internet Explorer and hence can't avail of ieSpell, here's something that'll be of interest. It occupies a spot on the system tray and emits a beep and flashes, whenever you make a spelling mistake on any online forms, while posting on the forums, and while creating quizzes. It also offers suggestions for alternative spellings. If it doesn't recognise a word you can add it to it's 'dictionary'.

Here's the programs main page: http://www.megspace.com/computers/tinyspell/

By the way, I found out about this from tellywellies' home page.