Another option for Windows users is a commercial product called
Spell Magic. It installs in Windows (not the browsers), but it can spell check in every browser I use. I like it because it does a good job checking, it can correct double words, and it's configurable. It also works better than any other universal spell checker I've tried.
I actually bought
Word Spring, which is Spell Magic plus a definition feature and thesaurus. But I never use the extra features because I don't like the way they work.