Apart from the obvious Google one (PR etc) I use IeSpell, a spell checker for the browser. It has options for true English and American English. I use it for posts on this board, but I have set it to true English and cannot be arsed to change it whenever I post on a mostly American board, which is why some of my posts have what you would consider as spelling mistakes. But it can be set to American English to those who want to look literate in the US (and look illiterate to the rest of the world
).
http://www.iespell.com/