It is Opera that scores highest for us in day to day use: combined with Gmail it has a very good built in spell checker.
If you have any degree of visual impairment or if like me you just want to read back what you have written in a different font size to spot typos, then the zoom in Opera is magic.
The range and number of suggestions for any typo is (at the moment) superior to any other browser we have found, and any page that has a muti-line text entry box will be automatically checked. In the latest version it checks both single line and multi line text entry areas.
Opera also give a much more comprehensive list of checked spellings:
It is easy to change the dictionary (via the option at the bottom of the spell check menu) to English:
I also like the lack of buttons and menus: Opera allows more
space for the important stuff. Its simple and really good.
Another nice feature of Opera is Opera Link. This allows you to share bookmarks and notes between different computers automatically.