i just do my best at spelling, when i am slipping (especially late at night) i usually come back and edit, or if it is very very close to proper and not in a thread that matters that much, you can sometimes let it slide :)
For IE there's iespell, for Firefox, there's spellbound.
iespell: www.iespell.com
spellbound: http://spellbound.sourceforge.net/
Both put icons on the toolbar and give you the ability to add words to a user dictionary.