EDIT: thanks for the reply! So, although this add-on is not a spell checker, it may be connected locally to one, a HUGE one, for that matter (about 180 MiB currently, plus some gibibytes/gigabytes, according to the developer). It's in Java, what means it could be downloaded for off-line use on different platforms (yes, I know Java is not as portable as advertised, but that's not the point...) I'm granting the five stars based on the on-line results, because my conscience won't let me use a spell checker that is bigger than all my text editors plus browsers plus pretty much every piece of software that can handle texts on my computer. I think it could be an amazing tool for publishers (although it would probably choke parsing texts like those by Philip K. Dick...)