It is a GUI to manage XMP metadata on Gnome. It is actualy a Nautilus extension. Only a few hundred lines of code, a Glade interface and exiftool, as backend. Simply, elegant, usable.
By default it only offers a few fields to edit, but what makes it more interesting, that the user can add custom fields! And it can edit multiple files.
It is an excellent tagging application! So simple, yet so powerful (exiftool can handle almost all image formats).