Opera has it and I think it's one of the best a webbrowser could add as a helpful feature. I know there is a plugin for Mozilla (dunno if and how it does work, cuz the page was always down, when I tried to download it and now I banned mozilla and stick to K-Meleon), so a port could be possible(?)...
I would like to see it in K-Meleon, because it would make another big step on its way to the best browser out there (IMO)