Emacs as a web browser's external editor
Since text editing in Emacs is better, extensions are developed for all major web browsers to use Emacs as their external editor. Mostly these extensions make use of Emacs daemon.
Iceweasel, Conkeror etc.
Conkeror comes with a small C program that enables external editing of form fields and page source in Emacs. Thus, no third-party extension/plugin is required.
Safari, Chrome etc.